Process for crosslinking and, if desired, foaming natural or synthetic homo- and/or copolymers

ABSTRACT

A process is provided for crosslinking, foaming, or both of polymers at temperatures above 150° C. using at least one azo ester and/or azo ether in combination with at least one crosslinking intensifier having at least two reactive carbon-carbon double or triple bonds.

BACKGROUND OF THE INVENTION

The invention pertains to the method, of crosslinking homo- and/orcopolymers using symmetric or unsymmetric azo esters or symmetric orunsymmetric azo ethers with the aid of crosslinking intensifiers.

U.S. Pat. No. 4,129,531 has as its goal the crosslinking and foaming ofpolymers with the aid of azo esters; U.S. Pat. No. 3,776,885 pertains tothe crosslinking of polymers with azo ethers.

SUMMARY OF THE INVENTION

The present invention is directed to a process of crosslinking orsimultaneously crosslinking and foaming a natural or synthetic homo- orcopolymer or a mixture thereof having --CH₂ -- and/or CH-groupscomprising: mixing with a 100 parts by weight of said polymer (i) 0.02to about 3.0% by weight of at least one member selected from the groupconsisting of an azo ester, an azo ether, and a mixture thereof and (ii)0.05 to about 10% by weight of at least one crosslinking intensifierhaving at least two reactive carbon-carbon double or triple bonds, andheating the polymer mixture at a temperature above 150° C. but not tothe point where the polymer will substantially degrade and at a pressureof from about 0 to about 300 psia until crosslinking or crosslinking andfoaming are effected.

DETAILED DESCRIPTION OF THE INVENTION

Within the framework of this invention it was found that combinations ofsmaller amounts of azo esters (e.g., Formula I below) or azo ethers(e.g., Formula II below) ##STR1## with crosslinking intensifiers (e.g.,triallyl cyanurate, triallyl trimellitate, etc.) are excellentlysuitable for crosslinking of polymers, and provide considerableadvantages, as described hereinafter, compared to the use of azo estersof azo ethers alone, thus in the absence of crosslinking intensifiers.

Unless otherwise indicated, percentages are percent by weight.

1. In the present invention a considerable reduction is possible in thequantity of azo ester or azo ether required to produce a degree ofcrosslinking while with azo esters or azo ethers alone the same degreeof crosslinking can only be obtained by using a considerably largeramount of these azo compounds. For example, in the crosslinking ofpolyethylene when using a combination of 0.6% azo ester (I) and 0.3%triallyl cyanurate, a crosslinking degree of 74% was achieved, whereaswhen using large amounts of the azo ester (I) alone, such as 1.5%, 2%,3%, 4% and 5%, crosslinking degrees of only 71%, 76%, 80%, 81% and 82%,respectively, were yielded.

It is even possible to reduce the quantity of azo compound below 0.1%,e.g., to 0.05% or 0.02%, and nevertheless achieve good degrees ofcrosslinking. For example, polyethylene at 200° C. was crosslinked tothe extent of 81% using a combination of 0.5% triallyl cyanurate and0.05% 2,2'-azo-bis-(2-acetoxy-4-methylpentane).

2. In accordance with this invention the combination of azo crosslinkersand crosslinking intensifiers can even give the possibility of achievinga good degree of crosslinking if the azo crosslinker alone produced nocrosslinking whatsoever. For example, ethylene-propylene copolymer(=EPR, saturated type) was crosslinked to the extent of 78% using acombination of 1.5% azo ester (I) and 0.7% triallyl cyanurate as acrosslinking intensifier, while no appreciable crosslinking was achievedwith the azo ester alone. Specifically, 1.5% azo ester (I) aloneresulted in a crosslinking degree of only 0.7%, and 5% of (I) gave acrosslinking degree of only 1.5%.

3. In accordance with this invention the combination of an azo compoundand a crosslinking intensifier permits crosslinking of sensitivepolymers and copolymers such as polypropylene. For example, 5% oftriallyl trimellitate and 1% of 2,2'-azo-bis-(2-acetoxy-4-methylpentane)were able to crosslink polypropylene to the extent of 67% at 200° C. and78% at 180° C., whereas no crosslinking occurs with the azo compoundalone and peroxide crosslinkers degrade polypropylene.

4. An additional advantage of the system in accordance with thisinvention as compared to the use of azo esters or azo ethers alone liesin the "kicker" effect of the crosslinking intensifier, which reducesthe start-up temperature by about 20° C.

5. Using the system in accordance with this invention it is possible,under unpressurized or practically unpressurized conditions (below 5 baror 72.5 psia) to obtain non-foamed, bubble-free polymer molded articles(e.g., pipes) with a good degree of crosslinking. In this caseparticularly favorable results are achieved if azo esters or azo ethersare used in very small amounts (about 0.02 to 0.2%) in the combinationin accordance with this invention.

6. As compared to peroxide type crosslinking agents, this inventionoffers the advantage that mixing with the polymer at higher temperaturesis possible without scorching, which is especially important in the caseof polymers with a high softening point, e.g., high density polyethyleneor polypropylene.

In accordance with this invention crosslinking can be carried out attemperatures above 150° C., wherein the upper limit depends on thethermal stability of the polymer.

The foaming ability of the azo esters or azo ethers, which give offmolar quantities of nitrogen upon thermal degradation, is also retainedwhen crosslinking intensifiers are used. If the crosslinking is carriedout in accordance with the invention under application of pressure, nofoaming takes place. When small amounts of azo compounds are used (forexample, less than 0.2%), the simultaneous use of crosslinkingintensifiers make it possible to perform bubble-free crosslinking evenunder nonpressurized conditions, as described hereinbefore.

The symmetric or unsymmetric azo esters or azo ethers used ascrosslinking agents are used individually or in combination in totalamounts of 0.02 to 3.0 wt-%, preferably 0.1 to 2.5 wt-%, especiallypreferably 0.4 to 1.5 wt-%, based on the polymer to be used.

It is also possible to combine one or more of the azo compounds used inaccordance with this invention with one or more crosslinking-promotingfree radical forming agents such as peroxides, azides and diazoacetates,wherein the total amounts of the crosslinking agent combinations fallwithin the ranges mentioned hereinbefore.

It is also possible to use the azo compounds in accordance with theinvention in themselves, or their combinations with one or more otherfree radical forming agents, together with foaming agents (e.g.,azodicarbonamide, azobisisobutyronitrile).

The amounts of crosslinking intensifiers which are used individually orin combination fall within the overall range of 0.05 to 10 wt-%,preferably from 0.1 to 5 wt-%, and especially preferably from 0.2 to2wt-%, based on the polymer to be crosslinked.

The relationship of the quantity of crosslinking intensifiers or theircombinations to the quantities of azo compound or their combinationswith other free radical forming agents can assume any desired value.Preference is given to weight ratios of crosslinking intensifiers to azocompound of 0.5 to 10:1, preferably 0.75 to 2:1 and especiallypreferably about 1:1. At such ratios the highest crosslinking degree ismade possible with a simultaneous marked decrease in the amount ofcrosslinking agent and crosslinking intensifier.

The sum of crosslinking intensifier and azo compound is not critical. Ingeneral it amounts to at least 0.4 wt-%, based on the polymer to becrosslinked and can be as high as 13.0%; preference is given to 0.5-7.5wt-% based on the polymer.

Obviously, instead of an individual crosslinking intensifier, anydesired mixtures of two or more crosslinking intensifiers can be used.

As the symmetric or unsymmetric azo esters or azo ethers, thosecustomarily employed for the crosslinking of polymers may be used, asdescribed for example in U.S. Pat. No. 4,129,531 and in U.S. Pat. No.3,776,885. The following preferred compounds of General Formula III(symmetric or unsymmetric) and IV (unsymmetric), individually or incombination, have proven particularly suitable: ##STR2##

In Formulas III and IV the substituents R, R¹, R², R⁴, R⁵ and R⁶ havethe following meanings:

R is selected from the group consisting of linear or branched alkyl of 1to 11 carbons, substituted or unsubstituted cycloalkyl of 5 to 6carbons, substituted or unsubstituted phenyl, substituted orunsubstituted aralkyl of 7 to 15 carbons, and R³ --CO--, wherein thesubstituent is alkyl of 1 to 10 carbons,

R³ is selected from the group consisting of H, linear or branched alkylof 1 to 10 carbons, substituted or unsubstituted cycloalkyl of 5 to 6carbons, substituted or unsubstituted phenyl, and substituted orunsubstituted aralkyl of 7 to 15 carbons, wherein the substituent isalkyl of 1 to 10 carbons,

R¹ and R² are independently selected from the group consisting of linearor branched alkyl of 1 to 10 carbons, substituted or unsubstitutedcycloalkyl of 5 to 6 carbons, substituted or unsubstituted phenyl, andsubstituted or unsubstituted aralkyl of 7 to 15 carbons, wherein thesubstituent is alkyl of 1 to 10 carbons,

R¹ R² C-- can join together to form a group selected from substituted orunsubstituted cycloalkyl of 5 to 6 carbons wherein the substitutent isalkyl of 1 to 10 carbons,

--OR and --R¹ can join together to form a member selected from the groupconsisting of --OCO--CH₂ --CH₂ -- and --OCO--CH₂ --CH₂ --CH₂ --,

R⁴, R⁵ and R⁶ are independently selected from the group consisting oflinear or branched alkyl of 1 to 10 carbons, substituted orunsubstituted cycloalkyl of 5 to 6 carbons, substituted or unsubstitutedphenyl, and substituted or unsubstituted aralkyl of 1 to 10 carbons,wherein the substituent is alkyl of 1 to 10 carbons,

R⁵ R⁶ C-- can join together to form a member selected from the groupconsisting of substituted or unsubstituted cycloalkyl of 5 to 6 carbonswherein the substituent is alkyl of 1 to 10 carbons.

In General Formula III, the groups R, R¹, R², R³, R¹ R² C and RR¹ on theleft hand side of the molecule are generally identical to thecorresponding groups on the right hand side of the molecule, but theymay also be different, and either one or several of the groups may bedifferent. Thus it is also possible that R--O-- on the left hand sidewill represent an ether group and on the right hand side an ester group(R³ --COO--), and thus a mixed azo-ester-ether will be present.

Preferred alkyl substituents in the aforementioned groups R, R¹, R², R³,R⁴, R⁵ and R⁶ are: C₁₋₆ alkyl, especially methyl, ethyl and propyl.

Examples of preferred groups R, R¹, R², R³, R⁴, R⁵ and R⁶ are: alkylgroups, such as C₁₋₆ alkyl, especially methyl, ethyl and propyl andaralkyl with C₁₋₁₀ alkyl groups such as benzyl or phenethyl.

Suitable symmetrical azo esters of Formula III are, for example:

2,2'-azo-bis-(2-acetoxypropane),

2,2'-azo-bis-(2-acetoxybutane),

2,2'-azo-bis-(2-acetoxy-3-methylbutane),

3,3'-azo-bis-(3-acetoxy-2,4-dimethylpentane),

2,2'-azo-bis-(2-acetoxy-4-methylpentane,

1,1'-azo-bis-(1-acetoxy-cyclohexane),

1,1'-azo-bis-(1-acetoxy-1-phenylethane),

1,1'-azo-bis-(1-acetoxy-2- or 3- or 4- or 2/3-or 2/4- or2/3/4-mixture-methyl-cyclohexane),

1,1'-azo-bis-(1-acetoxy-3,3,5-trimethyl-cyclohexane),

γ,γ'-azo-bis-(γ-valerolactone),

2,2'-azo-bis-(2-formyloxypropane),

2,2'-azo-bis-(2-formyloxybutane),

2,2'-azo-bis-2-formyloxy-4-methylpentane),

1,1'-azo-bis-(1-formyloxy-cyclohexane),

2,2'-azo-bis-(2-propionoxypropane),

1,1'-azo-bis-(1-propionoxy-cyclohexane),

2,2'-azo-bis-(2-benzoyloxypropane),

1,1'-azo-bis-(1-benzoyloxycyclohexane),

2,2'-azo-bis-(2-isobutyryloxybutane),

2,2'-azo-bis-(2-pivaloyloxypropane),

2,2'-azo-bis-(2-isobutyryloxy-4-metylpentane), and

1,1'-azo-bis-(1-isobutyryloxy-cyclohexane).

Suitable unsymmetric azo esters of Formula III are, for example:

2-[2'-acetoxypropyl-(2')-azo]-2-acetoxybutane,

1-[2'-acetoxybutyl-(2')-azo]-1-propionoxy-cyclohexane,

2-acetoxy-2'-propionoxy-2,2'-azo-bis-propane,

1-formyloxy-1'-acetoxy-2,2'-azo-bis-cyclohexane,

[2-acetoxy-4-methyl-pentyl-(2)][1'-acetoxy-cyclohexyl-(1')]-diazene.

Suitable symmetric azo ethers of Formula III are, for example:

2,2'-azo-bis-(2-methoxypropane),

2,2'-azo-bis-(2-methoxybutane),

2,2'-azo-bis-(2-methoxy-4-methylpentane),

1,1'-azo-bis-(1-methoxy-cyclohexane),

1,1'-azo-bis-(1-methoxy-3,3,5-trimethyl-cyclohexane),

2,2'-azo-bis-(2-ethoxypropane),2,2'-azo-bis-(2-ethoxy-4-methyl-pentane),1,1'-azo-bis-(1-ethoxy-cyclohexane),

1,1'-azo-bis-(1-ethoxy-3/4-mixture-methyl-cyclohexane),

2,2'-azo-bis-(2-isopropoxy-propane),

2,2'-azo-bis-(2-isopropoxybutane),

1,1'-azo-bis-(1-isopropoxy-cyclohexane),

2,2'-azo-bis-(2-propoxybutane),

2,2'-azo-bis-(2-butoxypropane),

2,2'-azo-bis-(2-butoxy-4-methylpentane),

1,1'-azo-bis-(1-butoxy-cyclohexane),

2,2'-azo-bis-(2-phenoxypropane), 1,1'-azo-bis-(1-phenoxy-cyclohexane).

Suitable unsymmetric azo ethers of Formula III are, for example:

2-[2'-methoxy-propyl-(2')-azo]-2-methoxybutane (equivalent to[2'-methoxypropyl-(2')]-[2-methoxybutyl-(2)]-diazene),1-[2'-methoxy-butyl-(2')-azo]-1-ethoxy-cyclohexane,2-methoxy-2'-ethoxy-2,2'-azo-bis-propane,1-methoxy-1'-isopropoxy-1,1'-azo-bis-cyclohexane,[2-ethoxy-4'-methylpentyl-(2')]-[1-ethoxy-cyclohexyl-(1)]-diazene,2-isopropoxy-2'-butoxy-2,2'-azo-bis-butane.

Suitable mixed azo ester ethers of Formula III are, for example:

2-propionoxy-2'-propoxy-2,2'-azo-bis-propane,

2-acetoxy-2-ethoxy-2,2'-azo-bis-butane,

2-formyloxy-2'-methoxy-2,2'-azo-bis-(4-methylpentane),

1-acetoxy-1'-methoxy-1,1'-azo-bis-cyclohexane,

1-formyloxy-1'-methoxy-1,1'-azo-bis-cyclohexane,

2-formyloxy-2'-propoxy-2,2'-azo-bis-butane,

2-acetoxy-2'-isopropoxy-2,2'-azo-bis-propane.

Suitable unsymmetric azo esters of Formula IV are, for example:

2-tert.-butylazo- (or tert.-amylazo- orcumylazo)-2-acetoxy-4-methyl-pentane, 2-tert.-butylazo-(ortert.-amylazo-or cumylazo)-2-propionoxy-butane,

1-tert.-butylazo (or tert.-amylazo-or cumylazo)-1-formyloxy-cyclohexane,2-tert.-butylazo- (or tert.-amylazo- orcumylazo)-2-isobutyryloxy-propane,

1-tert.-butylazo- (or tert.-amylazo- or cumylazo)-1-acetoxy-cyclohexane,γ-tert.-butylazo-(or tert.-amylazo- or cumylazo)-γ-valerolactone,

2-(1-methylcyclohexylazo)-2-acetoxybutane,1-(t-octylazo)-1-acetoxycyclohexane and 2-(t-hexylazo)-2-acetoxypropane.

Suitable unsymmetric azo ethers of Formula IV are, for example:

2-tert.-butylazo- (or tert.-amylazo-orcumylazo)-2-methoxy-4-methyl-pentane, 1-tert.-butylazo- (ortert.-amylazo- or cumylazo)-1-methoxy-cyclohexane, 2-tert.-butylazo- (ortert.-amylazo- or cumylazo)-2-ethoxy-butane, 1-tert.-butylazo- (ortert.-amylazo- or cumylazo)-1-isopropoxy-cyclohexane, 2-tert.-butylazo-(or tert.-amylazo- or cumylazo)-2-propoxybutane,

2-tert.-butylazo- (or tert.-amylazo-. or cumylazo)-2-butoxypropane,2-tert.-butylazo- (or tert.-amylazo- or cumylazo)-2-phenoxypropane,1-tert.-butyl-azo-(or tert.-amylazo-or cumylazo)-1-phenoxycyclohexane,

1-(1-methylcyclohexylazo)-1-methoxycyclohexane,

2-(t-octylazo)-2-methoxybutane and

2-(t-hexylazo)-2-methoxypropane.

Suitable crosslinking intensifiers include all compounds with at leasttwo reactive (=polymerizable) carbon-carbon double- and/or carbon-carbontriple bonds in the molecule, either individually or as a mixture.Suitable crosslinking intensifiers are, for example: di-, tri- orpolyallyl compounds, for example cyanurates (such as triallyl cyanurate,triallyl isocyanurate, diallyl isocyanurate), allyl esters of divalent,trivalent or polyvalent carboxylic acids (such as triallyl trimellitate,triallyl trimesinate, diallyl succinate, -glutarate, -adipate, diallylphthalate, diallyl diglycol dicarbonate), divalent, trivalent orpolyvalent N-allyl-substituted acid amides or imides (e.g.,N,N,N',N'-tetraallyl adipic acid diamide), allyl ethers on the basis ofdivalent, trivalent or polyvalent alcohols (such as trimethylolpropanetriallyl ether), allyl esters of dibasic, tribasic or polybasicinorganic acids (such as triallyl phosphate), allyl-substituted aminogroup-bearing s-triazines (such as2-butylamino-4,6-diallyl-oxy-s-triazine).

Di-, tri- or polymethallyl compounds corresponding to the abovementioned di-, tri- or polyallyl compounds, are for example trimethallylcyanurate.

Di-, tri- or polycrotyl compounds corresponding to the above mentioneddi-, tri- or polyallyl compounds, are for example tricrotyl cyanurate.

Di-, tri- or polymethacryl esters, are for example,

ethylene glycol-dimethacrylate,

hexanediol-(1,4)-dimethacrylate, trimethylolpropane-trimethacrylate.

Di-, tri- or polyacryl esters corresponding to the above mentioned di-,tri- or polymethacrylate esters, are for example diethylene glycoldiacrylate.

Polyenes (=polymers with double bonds), are for example polybutadienerubber (=1,2- or 1,4-polybutadiene).

Di-, tri- or polyvinyl compounds are for example divinylbenzene,butanediol-(1,4)-divinyl ether, trimethylolpropane-trivinyl ether,divinyl succinate, trivinyl isocyanurate, 1,2,4-trivinyl cyclohexane.

Compounds with triple bonds, are for example dipropargyl phthalate,tripropargyl trimellitate.

Di-, tri- or polymaleimides, are for example m- orp-phenylene-bis-(maleimide), 1,6-bis-maleimido-hexane,4,4'-methylene-bis-phenyl-maleimide, tris-(4-maleimido-phenyl)-methane,tris-, tetrakis- or pentakis-maleimides of Formula V ##STR3## in whichn=1,2 or 3.

Examples of compounds with different double or triple bonds in themolecule, are acrylate, allyl methacrylate, diallyl maleate, dipropargylmaleate; 1,3-bis-(allyloxy)-propyl-(2)-methacrylate,1-allyloxy-2-2-dimethylolbutane-dimethacrylate,2,2-bis-(allyloxymethyl)-butyl-(1)-methacrylate, N-allylmaleimide andN-vinylmaleimide.

All polymers or copolymers of compounds with --CH₂ -- and/or --CH--groups capable of crosslinking by free radical forming agents, andobtained either synthetically or naturally, as well as any desiredmixtures of these can be used in the practice of this invention. Suchcompounds include, for example, high pressure polyethylene of lowdensity, low pressure polyethylene of high and low density, chlorinatedpolyethylene, chlorosulfonated polyethylene, polypropylene,polybutylene-(1), poly-[4-methylpentene-(1)], polyvinyl chloride,polyvinyl acetate, polyacrylic acid esters, ethylene-vinyl acetatecopolymers, ethylene-propylene copolymers, ethylene-propylene-dieneterpolymers, ethylene-butylene copolymers, ethylene-propylene-butylenecopolymers, ethylene-vinyl chloride copolymers, vinyl chloride-vinylacetate copolymers, vinyl chloride-vinylidene chloride copolymers,ethylene-carbon monoxide [sic] copolymers, natural rubber(polyisoprene), polybutadiene, polychloroprene (neoprene), syntheticpolyisoprene, ethylene-propylene-ethylidene norbornene (orcyclopentadiene or hexadiene or butadiene) copolymer, butadiene-styrenecopolymer, butadiene-acrylonitrile copolymer,butadiene-styrene-acrylonitrile copolymer, styrene-isoprene blockpolymer, butyl rubber (isobutylene-isoprene copolymer), silicone rubber,saturated polyesters, polyamides, polyurethanes, polyethers andpolyacetals.

Examples of suitable organic peroxides include the following:

(a) dialkyl peroxides, e.g., dicumyl peroxide, tert.-butyl-cumylperoxide, α, α'-bis-(tert.butylperoxy)-1,4-(or 1,3)-diisopropylbenzene,2,5-bis-(tert.-butylperoxy)-2,5-dimethyl-hexane or -hexyne-(3),3-(tert.-butylperoxy)-3-phenyl-phthalide;

(b) perketals, e.g.,1,1-bis-(tert.-butylperoxy)-3,3,5-trimethylcyclohexane,4,4-bis-(tert.-butylperoxy)-pentanoic acid n-butylester;

(c) mixed dialkylperoxide perketals, e.g.,3,3,6,6,9,9-hexamethyl-1,2,4,5-tetraoxanonane;

(d) per-esters, e.g., tert.-butyl perbenzoate, tert.-butylper-3,5,5-tri-methylhexanoate, tert.-butylper-2-ethylhexanoate;

(e) monoperoxycarbonate esters, e.g., OO-tert.-butyl O-2-ethyl-hexylmonoperoxycarbonate, OO-tert.-butyl O-myristyl monoperoxycarbonate,decamethylene bis(OO-t-butyl monoperoxycarbonate);

(f) diacyl peroxide, e.g., 3,5,5-trimethylhexanoyl peroxide, lauroylperoxide, 2-methylbenzoyl peroxide, benzoyl peroxide,2,4-dichlorobenzoyl peroxide;

(g) ether peroxides, e.g., 2-tert.-butylperoxy-2-hexoxy-propane,2,5-bis-[2-butyloxypropyl-(2)-peroxy]-2,5-dimethylhexane;

(h) ketone peroxide-per-esters, e.g.,bis-[2-(2-ethylhexanoylperoxy)-butyl-(2)]-peroxide,bis-(2-lauroylperoxy)-butyl-(2)-peroxide,bis-(2-benzoyl-peroxy)-butyl-(2)-peroxide;

(i) tert.-alkylperoxyketone peroxides, e.g.,3,5-di-tert.-butylperoxy-3,5-dimethyl-1,2-dioxacyclopentane.

Examples of suitable organic azides include the following:

(a) di- or polysulfonyl azides, e.g., decane-1,10-bis-(sulfonazide),octadecane-1,9-18-tris-(sulfonazide), tris-(p- orm-azidosulfonylphenyl)-iso-cyanurate, benzene-1,3-disulfonazide;

(b) di- or polyazidoformates, e.g., tetramethylene-bis-(azidoformate),bis(β-azidoformyloxyethyl)terephthalate;

(c) aryl-bis-(azides), e.g., 2,6-bis-(4-azidobenzene)-cyclohexanone.

Suitable foaming agents include, for example, azodicarbonamide,azobisisobutyronitrile, azobisisobutyri acid diethyl ester,4,4'-azo-bis-(4-nitrilovalerianic acid), diazoaminobenzene,p,p'-hydroxybis-(bensenesulfonyl)hydrazide, benzenedisulfonyl hydrazide,diphenylsulfone-3,3'-disulfohydrazide,N,N-dinitroso-pentamethylenetetramine, p,p'-azo-bis-(benzenesulfonylsemicarbonamide), 5-morpholyl-(4)-1,2,3,4-thiatriazole,trihydrazinotriazine, NaHCO₃ +carbonic acid.

In accordance with this invention crosslinking can also be carried outin the presence of fillers (e.g., carbon black, calcium carbonate,talcum, calcium silicate, aluminum silicate (kaolin) or silicic acid(SiO₂)), glass fibers, pigments, antioxidants or other additives, suchas extenders, plasticizers, flame retardants, antistatic agents, andlubricants.

Certain polymers, such as high density polyethylene, if they containdouble (or triple) bonds in their molecules, may assert the function ofboth a polymer and a crosslinking intensifier.

The energy supply necessary for crosslinking can be accomplished (a) bydirect contact of the polymer to be crosslinked with a preheatedmaterial (e.g., steel, molten metal, molten salt, water, air, nitrogen),(b) by electromagnetic radiation (e.g., IR, UHF (=microwave), X-ray,gamma rays), or (c) by corpuscular radiation (e.g., alpha or beta rays(=electron beams)). It should be noted that certain types of energyradiation, e.g., gamma or beta rays, also bring about crosslinking inthe absence of crosslinking agents; however, the presence of freeradical forming agents and crosslinking intensifiers improve the degreeof crosslinking.

The invention will be further illustrated by means of the followingexamples.

Examples A. Crosslinking

The polymer to be crosslinked is homogeneously mixed (e.g., in a Banburymixer or on a mill) or in the form of a powder with the crosslinkingagent(s), if desired with addition of a solvent (acetone,dichloromethane), which is evaporated before the crosslinking process.Then the mixture is heated to crosslink the polymer and left at thecrosslinking temperature for a certain time.

B. Determination of the Degree of Crosslinking by Extraction

The crosslinked polymer, cut into strips after cooling, is packed intoan accurately weighed fine wire mesh, and after repeated weighing of thewire mesh and polymer, extracted for 1 hour with boiling xylene (80 mlxylene for 1 g crosslinked polymer) in a 100 ml Erlenmeyer flask underreflux. Then the wire mesh and the contents are washed with hot xylene,dried completely at 150° to 160° C. to dry the xylene (1 hr), and theweight loss, which corresponds to the polymer portion dissolved out,determined by weighing again. The difference from the weight of thepolymer before extraction indicates the fraction of undissolved polymer,its quantity expressed as a percent of the original total polymermixture, and the degree of crosslinking.

C. The following abbreviations are used for the initiators used in theexamples

(a) azo esters or azo ethers LUAZOAP=2,2'-azo-bis-(2-azetoxypropane)(=sym. azo ester) LUAZOABA=2,2'-azo-bis-(2-acetoxybutane)(=sym. azo ester) LUAZOAMP=2,2'-azo-bis-(2-acetoxy-4-methylpentane)(=sym. azo ester)

LUAZO AC=1,1'-azo-bis-(1-acetoxy cyclohexane)(=sym. azo ester)

LUAZO VL=γ,γ'-azo-bis-(γ-valerolactone)(=sym. azo ester)

sym. azo ether=1,1'-azo-bis-(1-methoxy-cyclohexane)

unsym. azo ester=1-tert.-butylazo-1-acetoxy-cyclohexane

unsym. azo ether=1-tert.-butylazo-1-methoxy-cyclohexane

(b) peroxides

peroctoate=tert.-butyl peroxy-2-ethylhexanoate (per ester)

perbenzoate=tert.-butyl peroxybenzoate (per ester)

LUPEROX 231=1,1-bis-(tert.-butyl peroxy)-3,3,5-trimethyl cyclohexane(perketal)

dicup=dicumyl peroxide (dialkyl peroxide)

LUPEROX 101=2,5-bis-(tert.-butylperoxy)-2,5-dimethylhexane (dialkylperoxide)

LUPEROX 130=2,5-bis-(tert.-butylperoxy)-2,5-dimethylhexyne-(3) (dialkylperoxide)

D. The crosslinking intensifiers are abbreviated as follows:

TAC=triallyl cyanurate

isoTAC=triallyl isocyanurate

diIso AC=diallyl isocyanurate2-butylam-DAC=2-n-butylamino-4,6-diallyloxy-s-triazine

2-octylam-DAC=2-n-octylamino-4,6-diallyloxy-s-triazine

2-stearylam-DAC=2-stearylamino-4,6-diallyloxy-s-triazine

TAPA=triallyl phosphate

TATM=triallyl trimellitate

DAP=diallyl phthalate

DADDC=diallyl diglycol dicarbonate

DAM=diallyl maleate

TRIDA=trimethylolpropane diallyl ether

TRIM=trimethylolpropane trimethacrylate

EDMA=ethylene glycol dimethacrylate

m-PBMI=m-phenylene-bis-(maleimide)

EXAMPLE 1

High pressure, low density polyethylene powder (melt index 70 g/10 min;density 0.918) is crosslinked for 40 min at 215° C. using the symmetricazo ester LUAZO AP (2,2'-azo-bis-(2-acetoxypropane) as crosslinkingagent with the aid of TAC (triallyl cyanurate) as crosslinkingintensifier under atmosphere pressure.

This example illustrates the rule that the most effective activation ofthe azo ester is achieved by a quantity of TAC (crosslinkingintensifier) equal to approximately half or double its amount.

It is noteworthy that 0.4% azo ester alone has a crosslinking degree of28%, but the combination of 0.4% azo ester+0.2% TAC gives a crosslinkingdegree of 74%, which is as high as in the case when 1.5% azo ester isused alone. It is even more remarkable that a combination of 0.8% azoester+0.6% TAC gives a crosslinking degree of 81%, quite as high as thatwhich can be achieved with 4% or 5% azo ester alone (81% and 82%)respectively.

All samples heated in the presence of LUAZO AP showed a fine-pore foamstructure.

    ______________________________________                                        Additives             Degree of Crosslinking                                  ______________________________________                                        (a) no additives           6%                                                 (b) 0.1% LUAZO AP         11%                                                     0.3% LUAZO AP         25%                                                     0.4% LUAZO AP         28%                                                     0.5% LUAZO AP         28%                                                     0.6% LUAZO AP         33%                                                     0.8% LUAZO AP         60%                                                     1% LUAZO AP           62%                                                     1.5% LUAZO AP         71%                                                     2% LUAZO AP           76%                                                     3% LUAZO AP           80%                                                     4% LUAZO AP           81%                                                     5% LUAZO AP           82%                                                 (c) 0.1% TAC              10%                                                     0.15% TAC             11%                                                     0.2% TAC              12%                                                     0.3% TAC              14%                                                     0.4% TAC              18%                                                     0.5% TAC              20%                                                     0.6% TAC              22%                                                     0.8% TAC              25%                                                     1% TAC                27%                                                     1.5% TAC              29%                                                     2% TAC                36%                                                 (d) 0.2% LUAZO AP + 0.15% TAC                                                                           48%                                                     0.2% LUAZO AP + 0.2% TAC                                                                            38%                                                 (e) 0.4% LUAZO AP + 0.1% TAC                                                                            61%                                                     0.4% LUAZO AP + 0.2% TAC                                                                            74%                                                     0.4% LUAZO AP + 0.3% TAC                                                                            74%                                                     0.4% LUAZO AP + 0.04% TAC                                                                           59%                                                 (f) 0.5% LUAZO AP + 0.25% TAC                                                                           64%                                                     0.5% LUAZO AP + 0.5% TAC                                                                            44%                                                     0.5% LUAZO AP + 1% TAC                                                                              73%                                                     0.5% LUAZO AP + 1.5% TAC                                                                            58%                                                     0.5% LUAZO AP + 2% TAC                                                                              50%                                                     0.5% LUAZO AP + 2.5% TAC                                                                            53%                                                     0.5% LUAZO AP + 3% TAC                                                                              53%                                                     0.5% LUAZO AP + 4% TAC                                                                              63%                                                 (g) 0.6% LUAZO AP + 0.3% Tac                                                                            74%                                                     0.6% LUAZO AP + 0.4% Tac                                                                            52%                                                     0.6% LUAZO AP + 0.6% Tac                                                                            52%                                                     0.6% LUAZO AP + 0.8% Tac                                                                            75%                                                     0.6% LUAZO AP + 1% Tac                                                                              76%                                                 (h) 0.8% LUAZO AP + 0.4% TAC                                                                            79%                                                     0.8% LUAZO AP + 0.6% TAC                                                                            81%                                                     0.8% LUAZO AP + 0.8% TAC                                                                            63%                                                     0.8% LUAZO AP + 1% TAC                                                                              79%                                                     0.8% LUAZO AP + 1.5% TAC                                                                            70%                                                 (i) 1% LUAZO AP + 0.1% TAC                                                                              64%                                                     1% LUAZO AP + 0.4% TAC                                                                              69%                                                     1% LUAZO AP + 1% TAC  68%                                                     1% LUAZO AP + 1.5% TAC                                                                              48%                                                     1% LUAZO AP + 2% TAC  71%                                                 (j) 2% LUAZO AP + 0.1% TAC                                                                              81%                                                     2% LUAZO AP + 0.4% TAC                                                                              71%                                                     2% LUAZO AP + 1% TAC  77%                                                     2% LUAZO AP + 1.5% TAC                                                                              63%                                                     2% LUAZO AP + 2% TAC  70%                                                 ______________________________________                                    

EXAMPLE 2

Ethylene-propylene copolymer, saturated type (AP rubber) with a Mooneyviscosity ML 1+4 (100° C.) of 45 is crosslinked for 40 min at 215° C.using the LUAZO AP as a crosslinking agent with the aid of TAC as acrosslinking amplifier in the presence and absence of carbon black as afiller under atmospheric pressure. The crosslinking and determination ofthe degree of crosslinking were carried out as described initially.However, the crosslinking agents were incorporated on a hot roller.

This example illustrates particularly clearly the great advantage ofcombined use of azo ester and crosslinking intensifer, since AP rubbershows no crosslinking with the azo ester alone. The experiment alsoshows that the filler, carbon black, does not interfere. All of thesamples (e), (f), (g), (h), (i) and (j) heated in the presence of LUAZOAP developed a fine-pore foam.

    ______________________________________                                        Azo ester and/or carbon black                                                                         Degree of                                             crosslinking intensifier                                                                              Crosslinking                                          ______________________________________                                        (a) no additive                0.7%                                           (b) 3% carbon black            2.8%                                           (c) 0.7% TAC                   7%                                             (d) 0.7% TAC + 3% carbon black 18%                                            (e) 1.5% LUAZO AP              0.7%                                           (f) 5% LUAZO AP                1.9%                                           (g) 1.5% LUAZO AP + 3% carbon black                                                                          0.9%                                           (h) 5% LUAZO AP + 3% carbon black                                                                            1.4%                                           (i) 1.5% LUAZO AP + 0.7% TAC   78%                                            (j) 1.5% LUAZO AP + 0.7% TAC + 3%                                                                            74%                                                carbon black                                                              ______________________________________                                    

EXAMPLE 3

Ethylene-vinyl acetate copolymer powder (EVA) with a vinyl acetatefraction of 33%, an ethylene fraction of 67% and a melt index of 25 g/10min (at 190° C.) is crosslinked for 40 min at 220° C. using the twosymmetrical azo esters, LUAZO AMP or LUAZO AP, as crosslinking agentswith the addition of TAC as a crosslinking amplifier under atmosphericpressure. To determine the degree of crosslinking, 1 hour extractionwith an 80-fold volume of boiling xylene was carried out twice (insteadof only once).

This example shows that with this polymer, which is more difficult tocrosslink, the maximum achievable degree of crosslinking is lower thanin Examples 1 or 2. However, in this case as well the simultaneous useof the crosslinking intensifier TAC presents distinct advantages.

All samples heated in the presence of LUAZO AP developed a fine-porefoam.

    ______________________________________                                                                 Degree of                                            Additives                crosslinking                                         ______________________________________                                        (A)    with LUAZO AMP                                                         (a)      without additives    6%                                              (b)      0.1% LUAZO AMP      13%                                                       0.2% LUAZO AMP      18%                                                       0.5% LUAZO AMP      19%                                                       1% LUAZO AMP        21%                                                       2% LUAZO AMP        28%                                                       4% LUAZO AMP        37%                                                       5% LUAZO AMP        38%                                              (c)      0.5% TAC             6%                                                       1% TAC               7%                                                       2% TAC               8%                                              (d)      1% LUAZO AMP + 0.5% TAC                                                                           39%                                                       1% LUAZO AMP + 1% TAC                                                                             50%                                                       1% LUAZO AMP + 2% TAC                                                                             54%                                              (B)    with LUAZO AP                                                          (a)      without additives    6%                                              (b)      0.1% LUAZO AP       10%                                                       0.2% LUAZO AP       12%                                                       0.5% LUAZO AP       19%                                                       1% LUAZO AP         21%                                                       2% LUAZO AP         33%                                                       4% LUAZO AP         44%                                                       5% LUAZO AP         49%                                              (c)      0.5% TAC             6%                                                       1% TAC               7%                                                       2% TAC               8%                                              (d)      0.5% LUAZO AP + 0.5% TAC                                                                          54%                                                       0.5% LUAZO AP + 1% TAC                                                                            34%                                              (e)      1% LUAZO AP + 0.5% TAC                                                                            53%                                                       1% LUAZO AP +  1% TAC                                                                             64%                                                       1% LUAZO AP + 2% TAC                                                                              62%                                              ______________________________________                                    

EXAMPLE 4

Low density, high pressure polyethylene powder (melt index 70 g/10 min;density 0.918) is crosslinked for 40 min at 215° C. using threesymmetrical azo esters (LUAZO AP, LUAZO AC, LUAZO VL) or an unsymmetricazo ester with TAC as a crosslinking intensifier under atmosphericpressure.

This example shows that the crosslinking degrees of 0.6% azo ester+0.3%TAC (81%, 82%) fall in the same order of magnitude as the crosslinkingdegrees of 2 to 3% azo ester alone. Thus, combination with half itsquantity of TAC makes it possible to reduce the azo ester quantity toabout 1/5 of its original value while achieving an equal degree ofcrosslinking.

All samples heated in the presence of the azo ester displayed afine-pore foam.

    ______________________________________                                                                 Degree of                                            Additives                Crosslinking                                         ______________________________________                                        (A)    with LUAZO AP                                                          (a)      0.6% LUAZO AP       47%                                                       2% LUAZO AP         77%                                                       3% LUAZO AP         84%                                              (b)      0.3% TAC            19%                                              (c)      0.6% LUAZO AP + 0.3% TAC                                                                          81%                                              (B)    with LUAZO AC                                                          (a)      0.6% LUAZO AC       14%                                                       2% LUAZO AC         64%                                                       3% LUAZO AC         75%                                              (b)      0.3% TAC            19%                                              (c)      0.6% LUAZO AC + 0.3% TAC                                                                          81%                                              (C)    with LUAZO VL                                                          (a)      0.6% LUAZO VL       30%                                                       2% LUAZO VL         89%                                                       3% LUAZO VL         85%                                              (b)      0.3% TAC            19%                                              (c)      0.6% LUAZO VL + 0.3% TAC                                                                          82%                                              (D)    with unsym. azo ester                                                  (a)      0.6% unsym. azo ester                                                                             31%                                                       2% unsym. azo ester 84%                                                       3% unsym. azo ester 84%                                              (b)      0.3% TAC            19%                                              (c)      0.6% unsym. azo ester +                                                                           81%                                                       0.3% TAC                                                             ______________________________________                                    

EXAMPLE 5

Low density, high pressure polyethylene powder (melt index 70 g/10 min;density 0.918) is crosslinked for 40 min at 205° C. using thesymmetrical azo ester LUAZO ABA (2,2'-azo-bis-(2-acetoxybutane) in thepresence of triallyl isocyanurate (IosTAC) as a crosslinking intensifierunder atmospheric pressure. The sample heated in the presence of LUAZOABA developed a fine-pore foam structure.

    ______________________________________                                                                 Degree of                                            Additives                crosslinking                                         ______________________________________                                        (a)   no additives           15%                                              (b)   0.6% LUAZO ABA         12%                                              (c)   0.3% IsoTAC            15%                                              (d)   0.6% LUAZO ABA + 0.3% IsoTAC                                                                         54%                                              ______________________________________                                    

EXAMPLE 6

Low density, high pressure polyethylene powder (melt index 70 g/10 min;density 0.918) is crosslinked for 40 min at 215° C. using thesymmetrical azo ester, LUAZO AP, as a crosslinking agent and TAC ascrosslinking intensifier in the presence of azodicarbonamide as afoaming agent at atmospheric pressure.

This example shows that the combination of azo compound+crosslinkingintensifier brings about good crosslinking even in the presence of afoaming agent; (b), (d) and (e) were foamed.

    ______________________________________                                                                 Degree of                                            Additives                crosslinking                                         ______________________________________                                        (a)   no additives           12%                                              (b)   0.6% LUAZO AP          19%                                              (c)   0.3% TAC               23%                                              (d)   0.6% LUAZO AP + 0.3% TAC                                                                             74%                                              (e)   0.6% LUAZO AP + 0.3% TAC + 5%                                                                        79%                                                    azodicarbonamide                                                        ______________________________________                                    

EXAMPLE 7

Low density, high pressure polyethylene powder (melt index 70 g/10 min;density 0.918) is crosslinked for 40 min at 215° C. using thesymmetrical azo ester LUAZO AP and LUAZO VL with the aid of variouscrosslinking intensifiers at atmospheric pressure.

All samples crosslinked in the presence of LUAZO AP and LUAZO VL werefine-pore foams.

    __________________________________________________________________________                                  Degree of crosslinking                          __________________________________________________________________________    (I)                                                                              with LUAZO AP                                                                    (a)                                                                             no additives           9%                                                   (b)                                                                             0.5% LUAZO AP         28%                                                     0.6% LUAZO AP         33%                                             (A)   (a)                                                                             0.2% 2-butylam-DAC (butylam)                                                                        16%                                                     0.3% 2-butylam-DAC (butylam)                                                                        18%                                                     0.5% 2-butylam-DAC (butylam)                                                                        19%                                                     1% 2-butylam-DAC (butylam)                                                                          28%                                                     2.3% 2-butylam-DAC (butylam)                                                                        38%                                                   (b)                                                                             0.5% LUAZO AP + 0.2% butylam                                                                        38%                                                     0.5% LUAZO AP + 0.5% butylam                                                                        53%                                                     0.5% LUAZO AP + 2.3% butylam                                                                        73%                                                   (c)                                                                             0.6% LUAZO AP + 0.3% butylam                                                                        78%                                             (B)   (a)                                                                             0.2% 2-octylam-DAC (octylam)                                                                        18%                                                     0.5% 2-octylam-DAC (octylam)                                                                        19%                                                     1% 2-octylam-DAC (octylam)                                                                          27%                                                     2.8% 2-octylam-DAC (octylam)                                                                        24%                                                   (b)                                                                             0.5% AP + 0.5% octylam                                                                              62%                                                     0.5% AP + 2.8% octylam                                                                              74%                                             (C)   (a)                                                                             0.3% diisoAC          20%                                                     0.6% AP + 0.3% diisoAC                                                                              70%                                             (D)   (a)                                                                             0.3% butanediol-(1,4)-divinyl ether                                                                 11%                                                   (b)                                                                             0.6% AP + 0.3% butanediol-(1,4)-divinyl                                                             60%                                                     ether                                                                 (E)   (a)                                                                             0.3% diallyl phthalate (DAP)                                                                        12%                                                   (b)                                                                             0.6% AP + 0.3% DAP    54%                                             (F)   (a)                                                                             0.3% TAPA             14%                                                   (b)                                                                             0.6% AP + 0.3% TAPA   65%                                             (G)   (a)                                                                             0.3% diallyl diglycol dicarbonate (DADDC) 25%                               (b)                                                                             0.6% AP + 0.3% DADDC  55%                                             (H)   (a)                                                                             0.3% diallyl maleate  13%                                                   (b)                                                                             0.6% AP + 0.3% diallyl maleate                                                                      55%                                             (II)                                                                             with LUAZO VL                                                                    (a)                                                                             no additives          10%                                                   (b)                                                                             0.6% LUAZO VL (=VL)   14%                                             (A)   (a)                                                                             0.3% TAC              18%                                                   (b)                                                                             0.6% VL + 0.3% TAC    78%                                             (B)   (a)                                                                             0.3% isoTAC           15%                                                   (b)                                                                             0.6% VL + 0.3% isoTAC 82%                                             (C)   (a)                                                                             0.3% diisoAC          20%                                                   (b)                                                                             0.6% VL + 0.3% diisoAC                                                                              76%                                             (D)   (a)                                                                             0.3% 2-butylam-DAC (=butylam)                                                                       18%                                                   (b)                                                                             0.6% VL + 0.3% butylam                                                                              61%                                             (E)   (a)                                                                             0.3% 2-octylam-DAC (=octylam)                                                                       18%                                                   (b)                                                                             0.6% VL + 0.3% octylam                                                                              67%                                             (F)   (a)                                                                             0.3% 2-stearylam-DAC (=stearylam)                                                                   10%                                                   (b)                                                                             0.6% VL + 0.3% stearylam                                                                            50%                                             (G)   (a)                                                                             0.3% TAPA             12%                                                   (b)                                                                             0.6% VL + 0.3% TAPA   64%                                             (H)   (a)                                                                             0.3% butanediol-(1,4)-divinyl ether                                                                  9%                                                   (b)                                                                             0.6% VL + 0.3% butanediol-(1,4)-divinyl                                                             28%                                                     ether                                                                 (I)   (a)                                                                             0.3% trimethylolpropane-diallyl                                                                      8%                                                     ether (TRIDA)                                                               (b)                                                                             0.6% VL + 0.3% TRIDA  18%                                             (J)   (a)                                                                             0.3% diallyl phthalate (DAP)                                                                        12%                                                   (b)                                                                             0.6% VL + 0.3% DAP    58%                                             (K)   (a)                                                                             0.3% diallyl diglycol dicarbonate                                                                   15%                                                     (DADDC)                                                                     (b)                                                                             0.6% VL + 0.3% DADDC  77%                                             (L)   (a)                                                                             0.3% diallyl maleate  13%                                                   (b)                                                                             0.6% VL + 0.3% diallyl maleate                                                                      80%                                             (M)   (a)                                                                             0.3% ethylene glycol dimethacrylate                                                                  7%                                                     (EDMA)                                                                      (b)                                                                             0.6% VL + 0.3% EDMA   43%                                             (N)   (a)                                                                             0.3% trimethylolpropane                                                                             20%                                                     trimethacrylate (TRIM)                                                      (b)                                                                             0.6% VL + 0.3% TRIM   54%                                             (O)   (a)                                                                             0.3% m-phenylene-bis-(maleimide)                                                                    71%                                                     (mPBMI)                                                                     (b)                                                                             0.6% VL + 0.3% m-PBMI 84%                                             __________________________________________________________________________

EXAMPLE 8

Hot vulcanizable silicone rubber (Silopren VS 60E 2302, containingmethyl and vinyl groups) is crosslinked for 40 min at 220° C. with anumber of azo esters and azo ethers as crosslinking agents with the aidof 0.3% TAC as a crosslinking intensifier at atmospheric pressure.Crosslinking agents and crosslinking intensifiers were incorporated intothe rubber on a hot roller at 70° C. The crosslinking and determinationof the degree of crosslinking were carried out as described initially(thus 1 g crosslinked rubber is extracted with 80 ml boiling xylene for1 hr.).

The azo compound containing specimens were fine-pore foams.

    ______________________________________                                        Additives             Degree of crosslinking                                  ______________________________________                                        (a) no additives           1%                                                 (b) 0.6% LUAZO AP + 0.3% TAC                                                                            74%                                                 (c) 0.6% LUAZO AMP + 0.3% TAC                                                                           77%                                                 (d) 0.6% unsym. azo ester + 0.3% TAC                                                                    64%                                                 (e) 0.6% sym. azo ether + 0.3% TAC                                                                      78%                                                 (f) 0.6% unsym. azo ether + 0.3% TAC                                                                    68%                                                 ______________________________________                                    

EXAMPLE 9

High pressure, low density polyethylene powder (melt index 70 g/10 min;density 0.918) is crosslinked for 40 min at various temperatures (180°,190°, 200° C.) with the aid of two symmetrical azo esters (LUAZO AMP andLUAZO and ABA) and an unsymmetrical azo ether as crosslinking agents andwith the aid of TAC as crosslinking intensifier under atmosphericpressure.

This example shows that in the case of lower temperatures as well (180°,190°, 200° C.), a considerable increase in the degree of crosslinking incomparison to the azo compound alone can be achieved with half thequantity of TAC (0.3%).

All samples crosslinked in the presence of the azo compound werefine-pore foams.

This example, Example 9, represents a supplement to Example 4.

    ______________________________________                                                              Degree of                                                                     crosslinking at                                         Additives               180°                                                                          190°                                                                          200°                             ______________________________________                                        without additives       5.2%   5.8%   6.5%                                    (A)  (a)   0.6% unsymmetric azo 11%  --   --                                             ether                                                                   (b)   0.3% TAC             10%  --   --                                       (c)   0.6% unsym. azo      52%  --   --                                             ether + 0.3% TAC                                                   (B)  (a)   0.6% LUAZO AMP       11%  49%  --                                       (b)   0.3% TAC             10%  15%  --                                       (c)   0.6% LUAZO AMP + 0.3% TAC                                                                          43%  64%  --                                  (C)  (a)   0.6% LUAZO ABA       --   --    8%                                      (b)   0.3% TAC             --   --   18%                                      (c)   0.6% LUAZO ABA + 0.3% TAC                                                                          --   --   65%                                 ______________________________________                                    

EXAMPLE 10

In all of the preceding examples the crosslinking was carried out in theabsence of pressure (thus at 1 atm), and a foamed material was alwaysobtained. In the present example, Example 10, polyethylene iscrosslinked under pressure. For this purpose, low density, high pressurepolyethylene powder (melt index 70 g/10 min; density 0.918), containing0.6% symmetrical azo ester LUAZO AP as crosslinking agent and 0.3% TACas crosslinking intensifier, incorporated homogeneously, was crosslinkedin a cylinder between two movable, precisely fitted pistons underpressure for 2 hrs at 215° C. The cylinders with the pistons werelocated between two steel plates, pressed together at the corners byfour screws with nuts. The pressure was generated by tightening thenuts. The nuts were retightened several times during the crosslinkingperiod.

In this manner a thin, white, non-foamed, but crosslinked polyethyleneplate was obtained, whose crosslinking degree was 84%.

EXAMPLE 11

Low density, high pressure polyethylene powder (melt index 70 g/10 min;density 0.918) was crosslinked for 40 min at 180° C. or 190° C. usingcombinations of two or three azo compounds with TAC as a crosslinkingintensifier under atmospheric pressure.

The samples crosslinked in the presence of the azo compounds werefine-pore foams.

    ______________________________________                                                               Degree of                                                                     crosslinking at                                        Additives                180° C.                                                                        190° C.                               ______________________________________                                            without additives            5.2%  5.8%                                       0.3% TAC                     10%   15%                                    (a) 0.3% sym. azo ether +                                                                                       8%   31%                                        0.3% unsym. azo ether                                                         0.3% sym. azo ether +                                                         0.3% unsym. azo ether +      53%   60%                                        0.3% TAC                                                                  (b) 0.3% LUAZO AMP +                                                                                            7%   44%                                        0.3% sym. azo ether                                                           0.3% LUAZO AMP +                                                              0.3% sym. azo ether +        45%   60%                                        0.3% TAC                                                                  (c) 0.3% LUAZO AMP +                                                                                           --    12%                                        0.3% unsym. azo ether                                                         0.3% LUAZO AMP +                                                              0.3% unsym. azo ether +      --    51%                                        0.3% TAC                                                                  (d) 0.2% LUAZO AMP + 0.2% LUAZO                                                   ABA + 0.2% unsym. azo        --    11%                                        ether                                                                         0.2% LUAZO AMP +                                                              0.2% LUAZO ABA +                                                              0.2% unsym. azo ether +            61%                                        0.3% TAC                                                                  (e) 0.2% LUAZO AMP + 0.2%                                                         sym. azo ether +             46%   --                                         0.2% unsym. azo ether                                                         0.2% LUAZO AMP +                                                              0.2% sym. azo ether +                                                         0.2% unsym. azo ether +      96%   --                                         0.3% TAC                                                                  (f) 0.2% sym. azo ether +                                                         0.2% unsym. azo ether +      --    72%                                        0.2% unsym. azo ester                                                         0.2% sym. azo ether +                                                         0.2% unsym. azo ether +                                                       0.2% unsym. azo ester +      --    62%                                        0.3% TAC                                                                  ______________________________________                                    

EXAMPLE 12

Low density, high pressure polyethylene powder (0.918) with a melt indexof 70 g/10 min. was crosslinked for 40 min at 185° C. using thesymmetrical azo ether 1,1'-azo-bis-(1-methoxy-cyclohexane) ascrosslinking agent with the aid of TAC as a crosslinking intensifier atatmospheric pressure.

This example is a supplement to Example 4 and Example 9, in which asymmetrical azo ether was lacking.

With quantities as small as 0.6% azo ether and 0.9% TAC, the same degreeof crosslinking (76%) is achieved with quantities as large as 3, 4, or5% azo ether alone. If the amount of TAC is increased to 1.2%, thedegree of crosslinking increases to 83%, which cannot even be achievedwith 5% azo ether. An increase in the quantity of triallyl cyanurate inthe combinations with azo ether in this example thus led to a distinctrise in the degree of crosslinking. All samples crosslinked in thepresence of the symmetrical azo ether were fine-pore foams.

    ______________________________________                                        Azo ether and/or cross-                                                       linking intensifier                                                                             Degree of crosslinking                                      ______________________________________                                        (a)   without additives                                                                              5%                                                     (b)   0.6% sym. azo ether                                                                           13%                                                           [1,1'-azo-bis-(1-meth                                                         oxy-cychlohexane)]                                                            1% sym. azo ether                                                                             31%                                                           2% sym. azo ether                                                                             64%                                                           3% sym. azo ether                                                                             75%                                                           4% sym. azo ether                                                                             76%                                                           5% sym. azo ether                                                                             76%                                                     (c)   0.2% TAC        12%                                                           0.3% TAC        15%                                                           0.6% TAC        12%                                                           0.9% TAC        17%                                                           1.2% TAC        19%                                                     (d)   0.6% sym. azo ether +                                                                         51%                                                           0.2% TAC                                                                      0.6% sym. azo ether +                                                                         58%                                                           0.3% TAC                                                                      0.6% sym. azo ether +                                                                         72%                                                           0.6% TAC                                                                      0.6% sym. azo ether +                                                                         76%                                                           0.9% TAC                                                                      0.6% sym. azo ether +                                                                         83%                                                           1.2% TAC                                                                ______________________________________                                    

EXAMPLE 13

Low density, high pressure polyethylene powder (0.918) with a melt indexof 70 g/10 min. was crosslinked using LUAZO AP as a crosslinking agentwith the aid of TAC as a crosslinking intensifier for 40 min. at 215° C.in the presence of the fillers carbon black, calcium carbonate oraluminum silicate, at atmospheric pressure.

While the fillers CaCO₃ and Al silicate reduce the degree ofcrosslinking, carbon black brings about a slight increase in the degreeof crosslinking. All samples crosslinked in the presence of the azoester were fine-pore foams.

    ______________________________________                                        Filler, azo ester, crosslinking                                                                          Degree of                                          intensifier                crosslinking                                       ______________________________________                                             without additives              6%                                             0.6% LUAZO AP (without filler)                                                                              37%                                             0.3% TAC (without filler)     14%                                             1.2% TAC (without filler)     26%                                             0.6% LUAZO AP +                                                                                             70%                                             0.3% TAC (without filler)                                                     0.6% LUAZO AP +                                                                                             75%                                             1.2% TAC (without filler)                                                (A)  with 3% carbon black                                                          (a) 3% carbon black           11%                                             (b) 3% carbon black +                                                                                       55%                                               0.6% LUAZO AP                                                               (c) 3% carbon black + 0.3% TAC                                                                              19%                                               3% carbon black + 1.2% TAC  28%                                             (d) 3% carbon black +                                                                                       76%                                               0.6% LUAZO AP + 0.3% TAC                                                      3% carbon black +                                                                                         80%                                               0.6% LUAZO AP + 1.2% TAC                                               (B)  with 3% calcium carbonate                                                     (CaCO.sub.3)                                                                  (a) 3% CaCO.sub.3              8%                                             (b) 3% CaCO.sub.3 + 0.6% LUAZO AP                                                                           11%                                             (c) 3% CaCO.sub.3 + 0.3% TAC   9%                                               3% CaCO.sub.3 + 1.2% TAC    13%                                             (d) 3% CaCO.sub.3 + 0.6% LUAZO AP +                                                                         60%                                               0.3% TAC                                                                      3% CaCO.sub.3 + 0.6% LUAZO AP +                                                                           61%                                               1.2% TAC                                                               (C)  with 3% aluminum silicate                                                     (Al--silicate)                                                                (a) 3% Al--silicate            6%                                             (b) 3% Al--silicate + 0.6%                                                                                  12%                                               LUAZO AP                                                                    (c) 3% Al--silicate + 0.3% TAC                                                                               8%                                               3% Al--silicate + 1.2% TAC  10%                                             (d) 3% Al--silicate + 0.6%                                                                                  32%                                               LUAZO AP + 0.3% TAC                                                           3% Al--silicate + 0.6%                                                                                    73%                                               LUAZO AP + 1.2% TAC                                                    ______________________________________                                    

EXAMPLE 14

High density granular polyethylene (0.950 to 0.953) with a melt index of1.7 to 2.3/10 min. was crosslinked with LUAZO AP as a crosslinking agentand TAC as a crosslinking intensifier for 40 min. at 220° C. underatmospheric pressure. Crosslinking and determination of the degree ofcrosslinking were performed as described initially, but the 1 hrextraction was performed with an 80-fold volume of boiling xylene, twice(instead of only once).

This example shows that high density polyethylene can also becrosslinked using a combination of azo compound+crosslinkingintensifier, and good degrees of crosslinking achieved. All samples werefine-pore foams.

    ______________________________________                                        Azo esters and crosslinking                                                   intensifiers        Degree of crosslinking                                    ______________________________________                                        0.4% LUAZO AP + 0.2% TAC                                                                          54%                                                       0.4% LUAZO AP + 0.4% TAC                                                                          66%                                                       0.4% LUAZO AP + 0.6% TAC                                                                          73%                                                       0.4% LUAZO AP + 0.8% TAC                                                                          74%                                                       0.6% LUAZO AP + 0.3% TAC                                                                          54%                                                       0.6% LUAZO AP + 0.4% TAC                                                                          63%                                                       0.6% LUAZO AP + 0.6% TAC                                                                          65%                                                       0.6% LUAZO AP + 0.8% TAC                                                                          68%                                                       0.6% LUAZO AP + 1% TAC                                                                            78%                                                       0.6% LUAZO AP + 1.2% TAC                                                                          79%                                                       0.8% LUAZO AP + 0.2% TAC                                                                          57%                                                       0.8% LUAZO AP + 0.4% TAC                                                                          58%                                                       0.8% LUAZO AP + 0.6% TAC                                                                          60%                                                       0.8% LUAZO AP + 0.8% TAC                                                                          74%                                                       0.8% LUAZO AP + 1% TAC                                                                            77%                                                       0.8% LUAZO AP + 1.2% TAC                                                                          78%                                                       0.8% LUAZO AP + 1.6% TAC                                                                          78%                                                       1% LUAZO AP + 0.2% TAC                                                                            54%                                                       1% LUAZO AP + 0.4% TAC                                                                            55%                                                       1% LUAZO AP + 0.5% TAC                                                                            62%                                                       1% LUAZO AP + 0.6% TAC                                                                            64%                                                       1% LUAZO AP + 0.8% TAC                                                                            74%                                                       1% LUAZO AP + 1% TAC                                                                              76%                                                       1% LUAZO AP + 1.2% TAC                                                                            77%                                                       1% LUAZO AP + 1.6% TAC                                                                            78%                                                       1% LUAZO AP + 2% TAC                                                                              80%                                                       ______________________________________                                    

EXAMPLE 15

A mixture of equal parts by weight of high pressure, low density,polyethylene powder (0.918) with a melt index of 70 g/10 min. andethylene-vinyl-acetate copolymer powder (EVA) with a melt index of 25g/10 min. (at 190° C.), containing 33% vinyl acetate and 67% ethylenewere crosslinked for 40 min at 220° C. using LUAZO AP as crosslinkingagent and TAC as crosslinking intensifier under atmospheric pressure.Crosslinking and determination of the degree of crosslinking werecarried out as described initially, but the 1-hr extraction with 80-foldvolume of boiling xylene was carried out twice (instead of just once).

This example shows that the mixture of polyethylene+EVA can becrosslinked even better by azo ester+TAC (degree of crosslinking 70%)than polyethylene alone (degree of crosslinking 62%). All crosslinkedsamples (j), k), l)) were fine-pore foams.

    ______________________________________                                        Polymer, azo ester, cross- Degree of                                          linking intensifier        crosslinking                                       ______________________________________                                        (a) polyethylene,                                                                             without additives     4%                                      (b) EVA,        without additives     4%                                      (c) polyethylene +                                                                            without additives     5%                                          EVA,                                                                      (d) polyethylene                                                                              +0.5% LUAZO AP       28%                                      (e) EVA         +0.5% LUAZO AP       24%                                      (f) polyethylene +                                                                            +0.5% LUAZO AP       10%                                          EVA                                                                       (g) polyethylene                                                                              +1% TAC              18%                                      (h) EVA         +1% TAC              10%                                      (i) polyethylene +                                                                            +1% TAC              21%                                          EVA                                                                       (j) polyethylene                                                                              +0.5% LUAZO AP +     62%                                                      1% TAC                                                        (k) EVA         +0.5% LUAZO AP +     57%                                                      1% TAC                                                        (l) polyethylene +                                                                            +0.5% LUAZO AP +                                                                                   70%                                          EVA         1% TAC                                                        ______________________________________                                    

EXAMPLE 16

High pressure, low density polyethylene powder (0.918) with a melt indexof 70 g/10 min. was crosslinked for 40 min. at 215° C. using variouscombinations of azo compound(s)+peroxide(s) as crosslinking agent andone or more different crosslinking intensifiers under atmosphericpressure.

This example shows that combinations of azo compound+crosslinkingintensifier+peroxide also yield good degrees of crosslinking. Thesamples were fine-pore foams.

    ______________________________________                                        Azo compound, peroxide,  Degree                                               crosslinking intensifier of crosslinking                                      ______________________________________                                        (a)  0.3% LUAZO AP + 0.3%        90%                                               peroctoate + 0.3% TAC                                                    (b)  0.3% LUAZO AP + 0.3%        73%                                               perbenzoate + 0.3% TAC                                                   (c)  0.3% LUAZO AP + 0.3%        80%                                               dicup + 0.3% TAC                                                         (d)  0.3% LUAZO AP + 0.3%        80%                                               LUP. 101 + 0.3% TAC                                                      (e)  0.3% LUAZO AP + 0.3%        76%                                               LUP. 130 + 0.3% TAC                                                      (f)  0.3% LUAZO AP + 0.3%        76%                                               LUP. 231 + 0.3% TAC                                                      (g)  0.3% LUAZO AP + 0.3%        77%                                               LUP. 231 + 0.3% butylam-DAC                                              (h)  0.3% LUAZO AP + 0.3% dicup +                                                  0.15% TAC + 0.15% diallyl   68%                                               maleate                                                                  (i)  0.3% LUAZO AP + 0.3%                                                          dicup + 0.15% TAC +         57%                                               0.15% isoTAC                                                             (j)  0.3% LUAZO AP + 0.3%                                                          LUP. 231 + 0.15% TAC + 0.15%                                                                              60%                                               diallyl maleate                                                          (k)  0.3% LUAZO AP +  0.3%                                                         LUP. 231 + 0.15% TAC        65%                                               + 0.15% isoTAC                                                           (l)  0.3% LUAZO AP + 0.3%                                                          LUP. 231 + 0.15% TAC +      62%                                               0.15% DADDC                                                              (m)  0.3% LUAZO AP + 0.15%                                                         peroctoate + 0.15% LUP.     70%                                               130 + 0.3% TAC                                                           (n)  0.3% LUAZO AP + 0.15%                                                         LUP. 130 + 0.15% LUP.       76%                                               231 + 0.3% TAC                                                           (o)  0.15% LUAZO AP + 0.15%                                                        LUAZO VL + 0.3% LUP. 231    60%                                               + 0.3% TAC                                                               (p)  0.15% LUAZO AP + 0.15% asym.                                                  azo ester + 0.15% peroctoate                                                                              79%                                               + 0.15% LUP. 130 + 0.3% TAC                                              ______________________________________                                    

EXAMPLE 17

Low density, high pressure polyethylene powder (0.918) with a melt indexof 70 g/10 min. was crosslinked for 40 min. at 220° C. using 0.6% of theazo ester LUAZO AP as crosslinking agent and 0.3% high molecular weight1,2-polybutadiene oil (Lithene AH) as crosslinking intensifier underatmospheric pressure. The samples crosslinked in the presence of LUAZOAP were fine-pore foams.

    ______________________________________                                        Azo ester and/or cross-                                                       linking intensifier  Degree of crosslinking                                   ______________________________________                                        (a)  without additives       12%                                              (b)  0.6% LUAZO AP           30%                                              (c)  0.3% 1,2-polybutadiene oil                                                                             9%                                              (d)  0.6% LUAZO AP + 0.3%                                                                                  56%                                                   1,2-polybutadiene oil                                                    ______________________________________                                    

EXAMPLE 18

A PVC paste, prepared from 70% paste PVC with a K value of 70 bytriturating with 30% dioctyl phthalate, was crosslinked with differentazo compounds and TAC for 40 min. at 180° C. and 190° C. underatmospheric pressure.

(a) Crosslinking: The PVC paste to be crosslinked was homogeneouslymixed with the crosslinking agent. Crosslinking was performed asinitially described.

(b) Determination of the degree of crosslinking by extraction: The PVC,cut into strips after cooling, was packed into an accurately weighedwire mesh basket, and after repeated weighing of the wire mesh and PVC,extracted for 1 hr with boiling tetrahydrofuran (85 ml tetrahydrofuranper 1 g PVC) in a 100 ml Erlenmeyer flask under reflux. Then the wiremesh containing the unextracted crosslinked PVC fraction was rewashedwith hot tetrahydrofuran, dried briefly at room temperature, then driedfor 1 hr at 140° C. in a drying oven. Then the weight loss correspondingto the dissolved out polymer fraction was determined by weighingrepeatedly. The difference from the weight of the polymer beforeextraction gives the undissolved polymer fraction, whose quantity,expressed as a percentage of the original total polymer amount,indicates the degree of crosslinking.

The crosslinking degrees in excess of 100% can be explained by thepartial incorporation of dioctyl phthalate into the polymer during thecrosslinking reaction. The samples were fine-pore foams.

(c) Crosslinking results:

    ______________________________________                                                             Degree of cross-                                                              linking based on                                                                PVC +                                                  Crosslinking agent and DOP +    PVC +                                         crosslinking intensifier                                                                             initiator                                                                              initiator                                     ______________________________________                                        (A)  non-stabilized PVC at 180° C.                                     (a)    0.1% LUAZO AMP +    75%      107%                                             0.05% TAC                                                                     0.1% LUAZO AMP +    70%      100%                                             0.1% TAC                                                                      0.1% LUAZO AMP +    57%       81%                                             0.2% TAC                                                               (b)    0.2% LUAZO AMP +    71%      101%                                             0.05% TAC                                                                     0.2% LUAZO AMP +    76%      107%                                             0.1% TAC                                                                      0.2% LUAZO AMP +    72%      102%                                             0.2% TAC                                                               (B)  stabilized PVC at 190° C.                                              stabilizer:                                                                     0.17% Ba/Cd stearate                                                          0.3% epozidized soy                                                           bean oil (Edenor 81)                                                          0.3% triphenyl phosphite                                                      based on 70% PVC powder +                                                     30% dioctyl phthalate                                                  (a)    0.2% LUAZO AP + 0.1% TAC                                                                          81%      115%                                      (b)    0.5% LUAZO AP + 0.1% TAC                                                                          71%      101%                                      ______________________________________                                    

EXAMPLE 19

High pressure, low density polyethylene powder (melt index 70 g/10 min;density 0.918) is crosslinked for 40 min. at 215° C. using crosslinkingagent combinations, all of which contain stearylazidoformate in additionto an azo ester and/or peroxide, with the simultaneous presence of TACor TAC+butylam-DAC as crosslinking intensifiers under atmosphericpressure.

This example shows that in the presence of stearylazidoformatecrosslinking degrees are achieved which are just as good as thoseachieved in its absence. It likewise shows that stearylazidoformate doesnot interfere in any combination.

The LUAZO VL containing samples were fine-pore foams.

    ______________________________________                                        Additives             Degree of crosslinking                                  ______________________________________                                        (a) 0.3% LUAZO VL + 0.3% TAC                                                                            58%                                                 (b) 0.6% LUAZO VL + 0.3% TAC                                                                            78%                                                 (c) 0.3% LUAZO VL + 0.3% stearyl-                                                                       76%                                                     azidoformate + 0.3% TAC                                                   (d) 0.3% LUAZO VL + 0.3% stearyl-                                                                       78%                                                     azidoformate + 0.15% TAC +                                                    0.15% butylam-DAC                                                         (e) 0.2% LUAZO VL + 0.2% LUP.                                                                           73%                                                     231 + 0.2% stearylazidoformate +                                              0.3% TAC                                                                  ______________________________________                                    

EXAMPLE 20

Five different types of synthetic rubber are crosslinked for 40 min at220° C. using four symmetrical azo esters, one symmetrical azo ether, anunsymmetrical azo ester and an unsymmetrical azo ether as crosslinkingagents with the aid of TAC as a crosslinking intensifier underatmospheric pressure. Crosslinking agents and crosslinking intensifierswere incorporated into the rubber on a hot roller at 70° C. Thecrosslinking and determination of the degree of crosslinking werecarried out as described initially (i.e., 1 g crosslinked rubber isextracted with 80 ml boiling xylene for 1 hr).

In all cases azo compound+TAC bring about good crosslinking, wherein thedegrees of crosslinking are considerably above the blank value (rubberheated with additive). The AP rubber Buna AP 201 cannot be crosslinkedby azo compounds alone; only the combination with a crosslinkingintensifier makes crosslinking possible. The azo compound containingsamples were fine-pore foams.

EXAMPLE 21

Low density, high pressure polyethylene powder (melt index 70 g/10 min;density 0.918) is crosslinked with 0.6% LUAZO AP as crosslinking agentwith the aid of 0.3% TAC as crosslinking amplifier in comparison to 0.6%LUAZO AP under atmospheric pressure (A) for 40 min at varioustemperatures and (B) at 215° C. for various lengths of time.

Part (A) of this example serves for determining the take offtemperature. It shows that the combination of 0.6% LUAZO AP+0.3% TACtakes off at 20° C. lower (at 200° C.) than 0.6% LUAZO AP alone (at 220°C.). It also shows that the maximum crosslinking degree of thecombination is higher than that of LUAZO AP alone.

Part (B) of this example serves for determining the minimum crosslinkingtime. It shows that the combination of 0.6% LUAZO AP+0.3% TAC requiresan approximately 5 minutes shorter minimum crosslinking time (ca. 20min) than LUAZO AP alone (ca. 25 min). It also shows that the maximumcrosslinking degree of the combination is higher than that of LUAZO APalone.

The samples crosslinked with LUAZO AP or with the combination LUAZOAP+TAC were fine-pore foams.

    __________________________________________________________________________                      Degree of crosslinking of the following rubbers             Azo compound and tri-                                                                           Buna Bina Buna Buna  Buna                                   allyl cyanurate   AP 201.sup.1                                                                       AP 241.sup.2                                                                       AP 258.sup.3                                                                       Huls 1502.sup.4                                                                     CB 10.sup.5                            __________________________________________________________________________    (a)                                                                             without additives                                                                             17%  28%  62%  88%   13%                                    (b)                                                                             0.6% LUAZO AMP + 0.3%                                                                         86%  87%  87%  --    99%                                      TAC                                                                         (c)                                                                             0.6% LUAZO AP + 0.3% TAC                                                                      79%  96%  95%  98%   99%                                    (d)                                                                             0.6% LUAZO VL + 0.3% TAC                                                                      --   --   89%  96%   --                                     (e)                                                                             0.6% LUAZO AC + 0.3% TAC                                                                      --   92%  --   --    96%                                    (f)                                                                             0.6% sym. azo ether +                                                                         --   94%  --   --    --                                       0.3% TAC                                                                    (g)                                                                             0.6% unsym. azo --   --   88%  --    --                                       ester + 0.3% TAC                                                            (h)                                                                             0.6% unsym. azo 81%  --   --   95%   --                                       ether + 0.3% TAC                                                            (i)                                                                             1% LUAZO AMP + 0.5% TAC                                                                       82%  --   --   --    --                                     __________________________________________________________________________     .sup.1 Buna AP 201 = ethylenepropylene rubber                                 .sup.2 Buna AP 241 = ethylenepropylene-ethylidene norbornene rubber, very     rapid                                                                         .sup.3 Buna AP 258 = ethylenepropylene-ethylidene norbornene rubber,          extremely rapid                                                               .sup.4 Buna Huls 1502 = styrenebutadiene rubber                               .sup.5 Buna CB 10 = polybutadiene rubber                                 

    ______________________________________                                                  Crosslinking degrees achieved with                                            0.6%                                                                          LUAZO            0.6% LUAZO AP                                                AP     0.3% TAC  + 0.3% TAC                                         ______________________________________                                        (A)  crosslinking                                                                  temperatures:                                                                 140° C.                                                                            0%       0%      0%                                               150° C.                                                                            0%       0%      0%                                               160° C.                                                                            0%       0%      0%                                               170° C.                                                                            0%       0%      0%                                               180° C.                                                                           6.9%     6.4%    7.1%                                              190° C.                                                                           5.9%     6.6%    6.9%                                              200° C.                                                                           5.6%     7.4%    35%                                               210° C.                                                                           8.7%     12%     71%                                               220° C.                                                                           32%      16%     80%                                               230° C.                                                                           56%      21%     77%                                               240° C.                                                                           63%      23%     73%                                          (B)  crosslinking                                                                  times:                                                                         5 minutes 3.6%     3.2%    4.3%                                              10 minutes 4.1%     3.9%    4.9%                                              15 minutes 4.0%     4.3%    5.1%                                              20 minutes 6.1%     7.1%    45%                                               25 minutes 41%      9.6%    68%                                               30 minutes 35%      12%     73%                                               35 minutes 40%      9.5%    68%                                               40 minutes 58%      8.1%    66%                                               45 minutes 39%      11%     68%                                               50 minutes 32%      11%     67%                                               55 minutes 30%      16%     83%                                               60 minutes 30%      17%     67%                                          ______________________________________                                    

EXAMPLE 22

Granular high density polyethylene (0.950-0.953) with a melt index of1.7 to 2.3/10 min. is crosslinked with LUAZO AMP or the unsymmetric azoether 1tert.-butylazo-1-methoxy-cyclohexane as crosslinking agent withthe aid of TAC as crosslinking intensifier in the Monsanto rheometer at195° C. until the maximum degree of crosslinking has been achieved (ca.2 hr). The measured values will be explained in the following table:

T₅ =scorche time=vulcanization starting time=time in min to reach 5% ofthe maximal crosslinking.

T₉₀ =time in min required to reach 90% of the maximal crosslinking.

TM₅₀ =torsional force applied at the maximal crosslinking to theoscillating disc of the Monsanto rheometer at a sensitivity setting of50. The magnitude of the TM₅₀ value makes a statement regarding thecrosslinking degree of polyethylene; the TM₅₀ values are relativequantities.

Since the inherent viscosity of the polyethylene mixture heated to 195°C. has a TM₅₀ value of 14, such a value of 14 corresponds to anon-crosslinked polyethylene.

This example shows that the viscosity (as an indirect measure of thedegree of crosslinking) in the presence of TAC is higher, and inaddition, bubble-free crosslinked products are obtained.

    ______________________________________                                                                            Appearance                                                                    of the                                    Azo compound, cross-                                                                          T.sub.5                                                                              T.sub.90     crosslinked                               linking intensifier                                                                           min.   min.   TM.sub.50                                                                           polyethylene                              ______________________________________                                        (a) non-crosslinked poly-                                                                         --     --   14    --                                          ethylene at the be-                                                           ginning of cross-                                                             linking                                                                   (b) 0.6% LUAZO AMP  3.5    54   52    foamed                                  (c) 0.6% unsym. azo 2.5    43   65    foamed                                      ether                                                                     (d) 0.6% LUAZO AMP +                                                                              3.8    62.5 59    bubble-free                                 0.6% TAC                                                                  (e) 0.6% unsym. azo 2.5    40.5 67    bubble-free                                 ether + 0.6% TAC                                                          ______________________________________                                    

EXAMPLE 23

Polypropylene powder (melt index 0.6 g/10 min at 230° C. and 2.16 kpload) is crosslinked for 40 min using (A) various amounts of LUAZO AMPwith cooperation of 5% triallyltrimellitate (TATM) as crosslinkingintensifier at 200° C., and (B) using 5% TATM+1% LUAZO AMP at differenttemperatures (175° to 205° C.) under nitrogen. The determination of thedegree of crosslinking is carried out as described initially, i.e., byextraction for 1 hr with an 80-fold volume of boiling xylene underreflux.

Experimental series (A) shows that crosslinking takes place only uponadding quantities of between 0.4% and about 2.5% LUAZO AMP, andexperimental series (B) shows that crosslinking no longer occurs above200° C.

    ______________________________________                                        Azo ester, crosslinking intensifier                                                                 Degree of crosslinking                                  ______________________________________                                        (A)  at 200° C. with various amounts                                        of LUAZO AMP:                                                            (a)    without additives      0.1%                                            (b)    0.09% LUAZO AMP        0.1%                                                   0.11% LUAZO AMP        0.0%                                                   0.5% LUAZO AMP         0.0%                                                   1% LUAZO AMP           0.0%                                                   2% LUAZO AMP           0.0%                                                   3% LUAZO AMP           0.0%                                                   4% LUAZO AMP           0.0%                                                   5% LUAZO AMP           0.0%                                            (c)    5% TATM + 0.09%        0.3%                                                   LUAZO AMP                                                              (d)    5% TATM + 0.11%        1%                                                     LUAZO AMP (equivalent to                                                      0.05% LUP. 101)                                                        (e)    5% TATM + 0.2%         8%                                                     LUAZO AMP                                                              (f)    5% TATM + 0.3%         14%                                                    LUAZO AMP                                                              (g)    5% TATM + 0.4%         60%                                                    LUAZO AMP                                                              (h)    5% TATM + 0.5%         67%                                                    LUAZO AMP                                                              (i)    5% TATM + 0.75%        66%                                                    LUAZO AMP                                                              (j)    5% TATM + 1%           67%                                                    LUAZO AMP                                                              (k)    5% TATM + 2%           48%                                                    LUAZO AMP                                                              (l)    5% TATM + 3%           24%                                                    LUAZO AMP                                                              (m)    5% TATM + 4%           5%                                                     LUAZO AMP                                                              (n)    5% TATM + 5%           0.2%                                                   LUAZO AMP                                                              (B)  at different temperatures                                                     (175° C. to 205° C.) with 1%                                    LUAZO AMP                                                                (a)    5% TATM + 1%           78%                                                    LUAZO AMP (at 175° C.)                                          (b)    5% TATM + 1%           78%                                                    LUAZO AMP (at 180° C.)                                          (c)    5% TATM + 1%           75%                                                    LUAZO AMP (at 185° C.)                                          (d)    5% TATM + 1%           74%                                                    LUAZO AMP (at 190° C.)                                          (e)    5% TATM + 1%           67%                                                    LUAZO AMP (at 200° C.)                                          (f)    5% TATM + 1%           3%                                                     LUAZO AMP (at 205° C.)                                          ______________________________________                                    

EXAMPLE 24

Polypropylene powder (melt index 0.6 g/10 min. at 230° C. and 2.16 kpload) is crosslinked using 1% LUAZO AMP with the aid of 5% of differentcrosslinking intensifiers for 40 min at 180° C. under nitrogen in theabsence and presence of 0.1% of the antioxidant,O,O'-di-tert.-butyl-p-cresol (TBK). The determination of the degree ofcrosslinking is carried out as described initially, i.e., by extractionfor 1 hr with an 80 fold volume of boiling xylene under reflux.

This example shows that in addition to triallyltrimellitate, other (butnot all) crosslinking intensifiers are suitable for the crosslinking ofpolypropylene, both in the absence and in the presence of anantioxidant.

    ______________________________________                                        Azo ester, crosslinking intensifier                                                                Degree of crosslinking                                   ______________________________________                                        (A)  with 1% LUAZO AMP + 5% of                                                     different crosslinking inten-                                                 sifiers:                                                                 (a)    5% TATM           78%                                                  (b)    5% TAC            71%                                                  (c)    5% TAPA           72%                                                  (d)    5% diallyl glutarate                                                                             0%                                                  (e)    5% diisoAC        49%                                                  (f)    5% DAP             7%                                                  (g)    5% TRIM           65%                                                  (h)    5% EDMA           63%                                                  (i)    5% m-PBMI         67%                                                  (B)  with 1% LUAZO AMP + 5% of                                                     different crosslinking inten-                                                 sifiers + 0.1% TBK anti-                                                      oxidant (Ant.):                                                          (a)    5% TATM + Ant.    18%                                                  (b)    5% TAC + Ant.     79%                                                  (c)    5% TAPA + Ant.    70%                                                  (d)    5% TRIM + Ant.    75%                                                  (e)    5% EDMA + Ant.    76%                                                  (f)    5% m-PBMI + Ant.  62%                                                  ______________________________________                                    

EXAMPLE 25

In this example, the symmetrical azo ester LUAZO AMP is used in verysmall amounts as free radical forming agent.

High pressure, low intensity polyethylene powder (melt index 70 g/10min; density 0.918) is crosslinked for 40 min. at 200° C. using acombination of very small amounts of the symmetrical azo ester LUAZO AMPas a free radical forming agent and TAC as crosslinking intensifier, atatmospheric pressure.

This example shows that in the presence of a crosslinkoing intensifiereven very small amounts of azo compound bring about good degrees ofcrosslinking. It is remarkable that better crosslinking takes place (77%compared to 81%) in the presence of smaller amount of TAC (0.5%) than inthe presence of larger amounts (1% or 2%) TAC.

The crosslinked product was not foamed.

    ______________________________________                                        Additives             Degree of crosslinking                                  ______________________________________                                        (a) without additives         6.5%                                            (b) 0.01% LUAZO AMP           6.0%                                                0.02% LUAZO AMP 6.3%                                                          0.03% LUAZO AMP 7.1%                                                          0.04% LUAZO AMP 8.0%                                                          0.043% LUAZO AMP 8.5%                                                         0.05% LUAZO AMP 9.9%                                                          0.1% LUAZO AMP 48%                                                            0.2% LUAZO AMP 61%                                                            0.5% LUAZO AMP 67%                                                            1% LUAZO AMP 75%                                                              2% LUAZO AMP 77%                                                          (c) 0.1% TAC                  10%                                                 0.5% TAC                  22%                                                 1% TAC                    25%                                                 2% TAC                    43%                                             (d) 0.1% TAC + 0.05 LUAZO AMP 41%                                             (e) 0.5% TAC + 0.043% LUAZO AMP                                                                             77%                                                 0.5% TAC + 0.05% LUAZO AMP                                                                              81%                                             (f) 1% TAC + 0.02% LUAZO AMP  46%                                                 1% TAC + 0.043% LUAZO AMP 70%                                                 1% TAC + 0.05% LUAZO AMP  73%                                             (g) 2% TAC + 0.05% LUAZO AMP  74%                                             ______________________________________                                    

EXAMPLE 26

In this example an unsymmetric azo ester and an unsymmetric azo etherare used in very small amounts as free radical forming agents.

Low density, high pressure polyethylene powder (melt index 70 g/10 min;density 0.918) is crosslinked for 40 min. at 220° C. using a combinationof TAC as crosslinking intensifier and very small amounts of theunsymmetric azo ester 1-tert.-butylazo-1-acetoxy-cyclohexane or theunsymmetric azo ether 1-tert.-butylazo-1-methoxy-cyclohexane underatmospheric pressure.

This example shows that the crosslinking intensifier TAC is activatedjust as well by small amount of unsymmetric azo compounds as by thesymmetrical azo ester LUAZO AMP of the preceding example, Example 25.

This crosslinked product was not foamed.

    ______________________________________                                                                 Degree of cross-                                     Additives                linking                                              ______________________________________                                        (a)    without additives     10%                                              (b)    0.5% TAC              26%                                                     1% TAC                36%                                              (A)  with unsym. azo ester:                                                   (a)    0.05% unsym. azo ester                                                                              11%                                                     0.1% unsym. azo ester 10%                                                     0.2% unsym. azo ester 13%                                                     0.5% unsym. azo ester 10%                                                     1% unsym. azo ester   63%                                                     2% unsym. azo ester   71%                                                     3% unsym. azo ester   83%                                              (b)    0.5% TAC + 0.05% unsym. azo ester                                                                   31%                                              (c)    1% TAC + 0.05% unsym. azo ester                                                                     51%                                                     1% TAC + 0.1% unsym azo ester                                                                       78%                                                     1% TAC + 0.2% unsym. azo ester                                                                      77%                                              (B)  with unsym. azo ether:                                                   (a)    0.05% unsym. azo ether                                                                              12%                                                     0.1% unsym. azo ether 17%                                                     0.2% unsym. azo ether 39%                                                     0.5% unsym. azo ether 79%                                                     1% unsym. azo ether   78%                                                     2% unsym. azo ether   71%                                                     3% unsym. azo ether   70%                                              (b)    0.5% TAC + 0.05% unsym. azo ether                                                                   80%                                              (c)    1% TAC + 0.05% unsym. azo ether                                                                     73%                                                     1% TAC + 0.1% unsym. azo ether                                                                      74%                                                     1% TAC + 0.2% unsym. azo ether                                                                      70%                                              ______________________________________                                    

EXAMPLE 27

In this example a symmetric azo ether and four symmetric azo esters areused in very small amounts as free radical forming agents at differenttemperatures.

Low density, high pressure polyethylene powder (melt index 70 g/10 min;density 0.918) is crosslinked for 40 min. at 190° C., 200° C., 210° C.,or 220° C. using combinations of TAC as crosslinking intensifiers andvery small amounts of the symmetrical azo compounds mentioned in thefollowing (azo ethers and azo esters) at atmospheric pressure.

This example shows that symmetrical azo ethers and azo esters when usedin very small amounts are also capable of activating crosslinkingintensifiers and supplying very good degrees of crosslinking, whichreach or exceed those of 2% to 5% of the azo compounds alone.

The crosslinked product was not foamed.

    ______________________________________                                                                     Degree                                                                        of                                                                            cross-                                           Additives                    linking                                          ______________________________________                                        (a)  at 190° C. [with 1,1'-azo-bis-(1-methoxy-                              cyclohexane)]:                                                           (a)    without additives         6%                                           (b)    0.5% TAC                  11%                                                 1% TAC                    18%                                          (c)    0.05% sym. azo ether      9%                                           (d)    1% TAC + 0.05% sym. azo ether                                                                           76%                                          (B)  at 190° C. [with 2,2'-azo-bis-(2-acetoxy-                              4-methyl-pentane)]:                                                      (a)    without additives         6%                                           (b)    0.5% TAC                  11%                                                 1% TAC                    18%                                          (c)    0.05% LUAZO AMP (symmetric azo ester)                                                                   7%                                           (d)    0.5% TAC + 0.05% LUAZO AMP                                                                              48%                                                 1% TAC + 0.05% LUAZO AMP  58%                                          (C)  at 200°  C. [with 2,2'-azo-bis-(2-acetoxy-4-                           methyl-pentane)]:                                                        (a)    without additives         6.5%                                         (b)    0.5% TAC                  22%                                          (c)    0.05% LUAZO AMP           8%                                                  0.1% LUAZO AMP            48%                                                 0.2% LUAZO AMP            61%                                                 0.5% LUAZO AMP            67%                                                 1% LUAZO AMP              75%                                                 2% LUAZO AMP              77%                                          (d)    0.5% TAC + 0.05% LUAZO AMP                                                                              81%                                          (D)  at 210° C. [with 2,2'-azo-bis-(2-acetoxy-                              propane)]:                                                               (a)    without additives         9%                                           (b)    0.5% TAC                  28%                                                 1% TAC                    35%                                          (c)    0.05% LUAZO AP (symmetric azo ester)                                                                    8%                                                  0.1% LUAZO AP (symmetric azo ester)                                                                     9%                                                  0.2% LUAZO AP (symmetric azo ester)                                                                     9%                                                  0.5% LUAZO AP (symmetric azo ester)                                                                     11%                                                 1% LUAZO AP (symmetric azo ester)                                                                       16%                                                 2% LUAZO AP (symmetric azo ester)                                                                       70%                                          (d)    1% TAC + 0.05% LUAZO AP   48%                                                 1% TAC + 0.1% LUAZO AP    70%                                          (E)  at 210° C. [with 2,2'-azo-bis-(2-acetoxy-                              butane)]:                                                                (a)    without additives         9%                                           (b)    0.5% TAC                  28%                                                 1% TAC                    35%                                          (c)    0.05% LUAZO ABA (symmetric azo ester)                                                                   8%                                                  0.1% LUAZO ABA (symmetric azo ester)                                                                    9%                                                  0.2% LUAZO ABA (symmetric azo ester)                                                                    9%                                                  0.5% LUAZO ABA (symmetric azo ester)                                                                    9%                                                  1% LUAZO ABA (symmetric azo ester)                                                                      59%                                                 2% LUAZO ABA (symmetric azo ester)                                                                      72%                                          (d)    0.5% TAC + 0.05% LUAZO ABA                                                                              42%                                                 1% TAC + 0.05% LUAZO ABA  71%                                          (F)  at 220° C. [with 1,1'-azo-bis-(1-acetoxy-                              cyclohexane)]:                                                           (a)    without additives         10%                                          (b)    0.5% TAC                  26%                                                 1% TAC                    36%                                          (c)    0.05% LUAZO AC (symmetric azo ester)                                                                    10%                                                 0.1% LUAZO AC (symmetric azo ester)                                                                     11%                                                 0.2% LUAZO AC (symmetric azo ester)                                                                     10%                                                 0.5% LUAZO AC (symmetric azo ester)                                                                     11%                                                 1% LUAZO AC (symmetric azo ester)                                                                       11%                                                 3% LUAZO AC (symmetric azo ester)                                                                       66%                                                 5% LUAZO AC (symmetric azo ester)                                                                       72%                                          (d)    1% TAC + 0.05% LUAZO AC   45%                                                 1% TAC + 0.1% LUAZO AC    67%                                          ______________________________________                                    

EXAMPLE 28

Low density, high pressure polyethylene powder (melt index 70 g/10 min;density 0.918) is crosslinked for 40 min. at 215° C. with LUAZO AP andLUAZO VL as crosslinking agents with the aid of TAC as crosslinkingintensifier in the presence of the antioxidants TBK antioxidant(O,O-di-tert.-butyl-p-cresol) and PADAT antioxidant(2-(p-phenylamino-anilino)-4,6-diallyloxy-s-triazine) at atmosphericpressure.

This example shows that good degrees of crosslinking are also achievedin the presence of antioxidants in the crosslinking of polyethylene.

All samples crosslinked in the presence of azo esters were fine-porefoams.

    ______________________________________                                                                     Degree                                                                        of                                                                            cross-                                           Additives                    linking                                          ______________________________________                                        (a) 0.3% TAC                     16%                                          (b) 0.6% LUAZO AP                33%                                          (c) 0.6% LUAZO AP + 0.3% TAC     73%                                          (d) 0.6% LUAZO AP + 0.3% TAC + 0.1% TBK-Ant.                                                                   51%                                          (e) 0.6% LUAZO AP + 0.3% TAC + 0.1%                                                                            80%                                              PADAT-Ant.                                                                (f) 0.6% LUAZO AP + 0.3% TAC + 0.3%                                                                            66%                                              PADAT-Ant.                                                                (g) 0.6% LUAZO VL                30%                                          (h) 0.6% LUAZO VL + 0.3% TAC     77%                                          (i) 0.6% LUAZO VL + 0.3% TAC + 0.1% TBK-Ant.                                                                   56%                                          (j) 0.6% LUAZO VL + 0.3% TAC + 0.1%                                                                            75%                                              PADAT-Ant.                                                                (k) 0.6% LUAZO VL + 0.3% TAC + 0.3%                                                                            74%                                              PADAT-Ant.                                                                ______________________________________                                    

EXAMPLES 29-38 General Experimental Procedures

The crosslinkable compositions used in Examples 29-38 were all preparedby the following procedure:

100 parts by weight of resin were fluxed in a C.W. BrabenderPlasti-corder mixer with a Roller-5 type mixing head. Unless otherwisespecified, a blade speed of 50 rpm and a mixing head temperature of 140°C. were used. The desired azo initiator and co-agent were then added tothe fluxed resin. The composition was then allowed to mix for three (3)minutes. The sample was then removed from the mixing head and quicklypressed into plaques, utilizing a room temperature Carver LaboratoryPress (Model C), and then the plaques were allowed to cool to roomtemperature.

Crosslinking determinations on the plaques were carried out using theMonsanto Oscillating Disk Rheometer and swell ratio measurements.

The Monsanto Rheometer test procedure consists of a uncured sampleenclosed under positive pressure in a heated die cavity contaning abiconical disk. The disk is oscillated (100 cycles/min) through an arcof 3°. The force or torque required to oscillate the disk is recorded asa function of time. This shear modulus is proportional to the extent ofcrosslinking and is a representation of the cure reaction. The shearmodulus increases as percent crosslinking increases. The test variablesrecorded from the rheometer curve were as follows:

M_(H) --Maximum torque (in-lbs), a measure of crosslinking attained.

M_(L) --Minimum torque (in-lbs), a measure of viscosity of the compoundand an indicator of scorch. Increased M_(L) values are indicative ofscorch.

M_(H) -M_(L) --Difference between maximum and minimum torque values.This value is useful in determining extent of crosslinking.

T_(C90) --Cure time (minutes), time to reach 90% of maximum torque asdefined by (M_(H) -M_(L)) 0.9+M_(L).

T_(S2) --Scorch time (minutes), time required for torque to increase twoinch-pounds above M_(L).

The cure temperature for examples 29-38 was 196° C.

Swell ratio measurements were obtained by immersing a 0.5 gram sample ofthe rheometer cured composition in closed 6 oz jars containing xylenefor 24 hrs in a circulating air oven at 110° C. Afterwhich the gel wasremoved and weighed using a pre-tared closed weighing bottle so thexylene present in the gel would not evaporate. The gel is then dried for24 hrs at 110° C. and then weighed. The following calculation is used todetermine the swell ratio. ##EQU1## K=ratio of density of polymer tosolvent at immersion temperature. For High Density Polyethylene K=1.17

For Ethylene-Propylene Diene (EPDM) rubbers K=0.99.

EXAMPLES 29-32

High Density Polyethylene (HDPE) resin (density 0.962) (Union CarbideDMDJ7008) with a melt flow index of 8.0 was utilized in the followingexamples listed in Table 1. The preparation and crosslinkingdetermination test followed are the same as in the general experimentalprocedures.

                  TABLE 1                                                         ______________________________________                                                        Examples                                                      Compositions (parts by weight)                                                                  29     30       31*  32*                                    ______________________________________                                        HDPE              100    100      100  100                                    2-t-Butylazo-2-methoxy-4-                                                                       2.0     2.0-    --   --                                     methylpentane                                                                 2,2'-Azobis(2-acetoxypropane)                                                                   --     --       2.3  2.3                                    Triallyl Cyanurate                                                                              --     2.0      --   2.0                                    M.sub.H -M.sub.L (in-lbs)                                                                        10     37       0    33                                    T.sub.C90 (mins)  13.0   6.0      --   39.0                                   T.sub.S2 (mins)   3.6    1.6      --   6.0                                    Swell Ratio       --     9.9      --   8.6                                    ______________________________________                                         *Examples 31 and 32 rheometer cured at 204° C.                    

Examples 29 and 31 show that 2-t-Butylazo-2-methoxy-4-methylpentane and2,2'-azobis (92-acetoxypropane) yield inefficient cures in thisparticular HDPE resin. Incorporation of a co-agent, i.e., triallylcyanurate, yield significant improvements, over the non co-agent curesystems as indicated by examples 30 and 32.

High Density Polyethylene (HDPE) resin (density 0.960) (PhillipsPetroleum Company) with a melt flow index of 30.0 was utilized in thefollowing examples listed in Table 2. The preparation and crosslinkingdetermination tests followed are the same as in the General ExperimentalProcedures.

                  TABLE 2                                                         ______________________________________                                                       Examples                                                       Compositions (parts by weight)                                                                 33       34     35*    36*                                   ______________________________________                                        HDPE             100      100    100    100                                   2-t-Butylazo-2-methoxy-4-                                                                       2.0     2.0    --     --                                    methylpentane                                                                 2.2'-Azobis(2-acetoxypropane)                                                                  --       --      2.3   2.3                                   Triallyl Cyanurate                                                                             --       2.0    --     2.0                                   M.sub.H -M.sub.L (in-lbs)                                                                       46       78     40     69                                   T.sub.C90 (mins) 11.0     7.5    53.0   33.0                                  T.sub.S2 (mins)   2.3     1.8     6.0   6.0                                   Swell Ratio      23.0     5.9    25.0   6.2                                   ______________________________________                                         *Examples 35 and 36 rheometer cured at 204° C.                    

The incorporation of a co-agent, e.g., triallyl cyanurate, significantlyimproves the crosslink density as indicated by the higher M_(H) -M_(L)values and lower swell ratio's for examples 34 and 36 as compared toexamples 33 and 35.

Ethylene-Propylene-Diene (EPDM) rubber (specific gravity 0.86) with aMooney Viscosity (ML-8, 100° C.) of 30 and a 5-ethylidene-2-norbornenecontent of 4-5% was utilized in the following examples listed in Table3. The mixing temperature and the blade speed were room temperature and30 rpm, respectively and the swelling drying temperatures were 25° C.and 60° C., respectively, otherwise the same procedure as described inthe General Experimental Procedure was followed.

                  TABLE 3                                                         ______________________________________                                                            Examples                                                  Compositions (parts by weight)                                                                      37     38                                               ______________________________________                                        EPDM                  100    100                                              2-t-Butylazo-2-methoxy-4-methyl-                                                                    2.0    2.0                                              pentane                                                                       Triallyl Cyanurate    --     2.0                                              M.sub.H -M.sub.L (in lbs)                                                                            39     72                                              T.sub.C90 (mins)      17.0   11.0                                             T.sub.S2 (mins)       2.0    1.8                                              Swell Ratio           9.0    4.7                                              ______________________________________                                    

Incorporation of a co-agent, i.e., triallyl cyanurate, significantlyincreased the torque values and lowered the swell ratio, indicating ahigh crosslink density for example 38 as compared to example 37.

A resin blend of polyvinyl chloride and chlorinated polyethylene (BlaneNo. 79-105E, Blane Division of Reichhold Chemicals Inc.) was utilized inthe following example listed in Table 4. The mixing temperature was 160°C., otherwise the same procedure as described in the GeneralExperimental Procedure was followed.

                  TABLE 4                                                         ______________________________________                                                               Example                                                Composition (parts by weight)                                                                        39                                                     ______________________________________                                        Blane No. 79-105E      100                                                    2-t-Butylazo-2-methoxy-4-methylpentane                                                               0.5                                                    Triallyl Cyanurate     3.5                                                    M.sub.H -M.sub.L (in lbs)                                                                             36                                                    T.sub.C90 (mins)       32.0                                                   T.sub.S2 (mins)        1.8                                                    ______________________________________                                    

Blends of Polypropylene (Density of 0.903), melt flow index of 4.0) andethylene-propylene diene (EPDM) rubber with a Mooney Viscosity (ML-8,100° C.) of 30 and a 5-ethylidene-2-norbornene content of 4-5% werecrosslinked with (1) 2-t-Butylazo-2-methoxy-4-methylpentane and (2)

2,2'-Azobis(2-acetoxypropane) as well as a mixture of both azo-esterinitiators. With only a change in the mixing temperature to 160° C. andthe rheometer cure temperature to 218° C., the examples, in Table 5,were prepared and Monsanto rheometer tested as described in the GeneralExperimental Procedure.

                  TABLE 5                                                         ______________________________________                                                         Example                                                      Compositions (parts by weight)                                                                   40      41    42     43                                    ______________________________________                                        Polypropylene      25      25    25     25                                    Ethylene-Propylene Diene Rubber                                                                  75      75    75     75                                    2-t-Butylazo-2-methoxy-4-methyl-                                                                 2.0     2.0   --     1.0                                   pentane                                                                       2,2'-Azobis(2-acetoxypropane)                                                                    --      --    2.0    1.0                                   Triallyl Cyanurate --      2.0   2.0    2.0                                   M.sub.H -M.sub.L (in lbs)                                                                        10      20    38     27                                    T.sub.C90 (mins)   4.0     3.2   30.5   22.5                                  T.sub.S2 (mins)    1.3     1.1   6.0    1.6                                   ______________________________________                                    

The results illustrate that the resin blend is crosslinked and furtherthat crosslinking co-agents, i.e., triallyl cyanurate, enhance thecrosslinking reaction as illustrated in examples 40-42.

Due to their inherent chemical inertness, different azo initiators canbe blended to obtain specific processing advantages. Thus, by using amixture of azo initiators, example 43, one can extend the scorch time orincrease the rate of cure.

Typical commercial organic peroxide crosslinking agents [dicumylperoxide; 1,4 and 1,3-di(2-t-butylperoxyisopropyl) benzene and2,5-di(t-butylperoxy)-2,5-dimethylhexane] were run under similarconditions as examples 40-42. The organic peroxide cure systems yieldedno measurable torque values even in the presence of triallyl cyanurate.

What is claimed:
 1. A process of crosslinking a polymer select from thegroup consisting of high pressure polyethylene of low density, lowpressure polyethylene of high and low density, chlorinated polyethylene,chlorosulfonated polyethylene, polypropylene, polybutyl-(1),poly-[4-methylpentene-(1)], polyvinyl chloride, polyvinyl acetate,polyacrylic acid esters, ethylene-vinyl acetate copolymers,ethylene-propylene butylene copolymers, ethylene-propylene-dieneterpolymers, ethylene-butylene copolymers, ethylene-propylene butylenecopolymers, ethylene-vinyl chloride copolymers, vinyl chloride-vinylacetate copolymers, vinyl chloride-vinylidene chloride copolymers,ethylene-carbon monoxide copolymers, natural rubber (polyisoprene),polybutadiene, polychloroprene (neoprene), synthetic polyisoprene,ethylene-propylene-ethylidene norbornene (or cyclopentadiene orhexadiene or butadiene) copolymer, butadiene-styrene copolymer,butadiene-acrylonitrile copolymer, butadiene-styrene-acrylonitrilecopolymer, styrene-isoprene block polymer, butyl rubber(isobutylene-isoprene copolymer), silicone rubber, saturated polyesters,polyamides, polyurethanes, polyethers, and polyacetals comprising:mixingwith a 100 parts by weight of said polymer (i) from about 0.02 to about3.0% by weight of at least one member selected from the group consistingof an azo ester, an azo ether, and a mixture thereof wherein the azoester and azo ether have the following formulas ##STR4## wherein: R isselected from the group consisting of linear or branched alkyl of 1 to11 carbons, substituted or unsubstituted cycloalkyl of 5 to 6 carbons,substituted or unsubstituted phenyl, substituted or unsubstitutedaralkyl of 7 to 15 carbons, and R³ --CO--, wherein the substituent isalkyl of 1 to 10 carbons, R³ is selected from the group consisting of H,linear or branched alkyl of 1 to 10 carbons, substituted orunsubstituted cycloalkyl of 5 to 6 carbons, substituted or unsubstitutedphenyl, and substituted or unsubstituted aralkyl of 7 to 15 carbons,wherein the substituent is alkyl of 1 to 10 carbons, R¹ and R² areindependently selected from the group consisting of linear or branchedalkyl of 1 to 10 carbons, substituted or unsubstituted cycloalkyl of 5to 6 carbons, substituted or unsubstituted phenyl, and substituted orunsubstituted aralkyl of 7 to 15 carbons, wherein the substituent isalkyl of 1 to 10 carbons, R¹ R² C-- can join together to form a groupselected from substituted or unsubstituted cycloalkyl of 5 to 6 carbonswherein the substituent is alkyl of 1 to 10 carbons, --OR and --R¹ canjoin together to form a member selected from the group consisting of--OCO--CH₂ --CH₂ -- and --OCO--CH₂ --CH₂ --CH₂ --, R⁴, R⁵ and R⁶ areindependently selected from the group consisting of linear or branchedalkyl of 1 to 10 carbons, substituted or unsubstituted cycloalkyl of 5to 6 carbons, substituted or unsubstituted phenyl, and substituted orunsubstituted aralkyl of 1 to 10 carbons, wherein the substituent isalkyl of 1 to 10 carbons, and R⁵ R⁶ C-- can join together to form amember selected from the group consisting of substituted orunsubstituted cycloalkyl of 5 to 6 carbons wherein the substituent isalkyl of 1 to 10 carbons and (ii) from about 0.05 to about 10% by weightof at least one crosslinking intensifier having at least two relativecarbon-carbon double or triple bonds, and heating the polymer mixture ata temperature above about 150° C. but not to the point where the polymerwill substantially degrade and at a pressure of from about atmosphericto about 300 psia until crosslinking is effected.
 2. The process ofclaim 1 wherein at least one other free radical crosslinking agent isincluded.
 3. The process of claim 2 wherein the at least one other freeradical crosslinking agent is selected from organic peroxides with theexception of hydroperoxides and per-acids.
 4. The process of claim 2wherein the at least one other free radical crosslinking agent isselected from organic azides.
 5. The process of claim 1 wherein thereaction is carried out in the presence of additional foaming agents. 6.The process of claim 1 wherein the reaction is carried out in thepresence of fillers.
 7. The process of claim 1 wherein the reaction iscarried out in the presence of antioxidants.
 8. The process of claim 1wherein the polymer is a polymer blend selected from the groupconsisting of (i) polypropylene and ethylene-propylene diene rubber and(ii) polyvinyl chloride and chlorinated polyethylene.
 9. The process ofclaim 1 wherein the intensifier is selected from the group consisting ofpolyallyl cyanurates, polyallyl esters of polyvalent carboxylic acids,polyvalent N-allyl-substituted acid amides or imides, polyallyl ethersof polyvalent alcohols, polyallyl esters of polybasic inorganic acids,and polyallyl substituted amino group-bearing s-triazines.
 10. Theprocess of claim 9 wherein the intensifier of polyallyl cyanurates isselected from the group consisting of triallyl cyanurate, triallylisocyanurate, and diallyl isocyanurate.
 11. The process of claim 9wherein the intensifier of polyallyl esters is selected from the groupconsisting of triallyl trimellitate, triallyl trimesinate, diallylsuccinate, diallyl glutarate, diallyl adipate, diallyl phthalate,diallyl diglycol dicarbonate.
 12. The process of claim 9 wherein theintensifier is polyvalent N-allyl-substituted acid amides or imides isN,N,N',N'-tetraallyl adipic acid diamide.
 13. The process of claim 9wherein the intensifier of polyallyl ethers of polyvalent alcohols isselected from the group consisting of trimethylolpropane and triallylether.
 14. The process of claim 9 wherein the intensifier of polyallylesters of polybasic inorganic acids is triallyl phosphate.
 15. Theprocess of claim 9 wherein the intensifier of polyallyl substitutedamino group-bearing s-triazines is2-butylamino-4,6-diallyl-oxy-s-triazine.
 16. The process of claim 1wherein (i) is from about 0.02 to about 0.2% by weight and the polymermixture is heated at atmospheric pressure wherein non-foamed,bubble-free polymer is produced.
 17. The process of claim 1 wherein (i)is from about 0.2 to about 3.0% by weight and the polymer mixture isheated at a pressure from about 72.5 to about 300 psia whereinnon-foamed, bubble-free polymer is produced.
 18. The process of claim 1wherein (i) is from about 0.2 to about 3.0% by weight and the polymermixture is heated while slowly raising the temperature at atmosphericpressure wherein non-foamed, bubble-free polymer is produced.